Cylinder compression can store energy. Drive chain - whatever it is, even a simple shaft - can also store energy. Look up the concept of "torsion spring". I've been hurt by energy stored in torsion springs, even using hand tools on ordinary bolts (play stupid games, win stupid prizes - that was me)

Yeah, well "could" is the whole point. Clear the jam with a stick - you never know, might save someone's finger once in a while.

Mechanic all my life. Lew is correct. It is nothing like a wind up toy.

The pressure will be relieved after the blade is released, less than 1cm of movement. it will not suddenly spin, I'm a landscaper, I do this for a living. I can still count to 10.

I've seen a lawnmower blade spin round quite a lot after being freed from a jam. Don't ask me about the details, I was a young child. There's always risk with a jammed mechanism, which is why I say "Use a stick!" in the case of lawnmowers.

Force can be stored in a rubber band, or a spring, or many other things. That is because they are pulled (elongated) in the case of a rubber band, or wound up, in the case of a spring. There are no springs or rubber bands, or anything like them in a lawn motor. The blades are connected to a drive shaft, which does not store energy. It turns and moves the connected blades when the motor is on. When the motor is off, there is no power to move the blades.

Of course a rotating drive shaft can store energy - look up the concept of "torsion spring". Any bit of any mechanism in real life can store energy: all materials are elastic in all directions, and all can store energy in tension, compression, or torsion. Any jammed mechanism has the *potential* to *maybe* have energy stored in it which might be a bother when released. Such stored energy has in fact walloped me from time to time - never worse than cuts or bruises, mostly down to luck rather than anything else. Don't think that your ignorance of reality will save you from the consequences of stored energy, if you should get unlucky.

The stick falls because gravity is acting on it, this isn’t a valid analogy. When the motor stops the pressure stops. There is no stored energy that could move the blades.

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@nola momma: the issue is that the motor is driving a mechanism. If that mechanism jams, then there could be energy stored up as in a spring (elastic energy). This is 100% normal and expected in powered machinery. Such stored energy in mechanical systems has sprung back and walloped me on more than one occasion. The energy can be stored in all sorts of ways - a twisted shaft, a stressed lever, compressed gas, anything.

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Why on earth were you downvoted for a rational explanation? Here's one to balance it out.

No it's not "finishing the cycle" any pressure will be relieved after a tiny amount of movement. It won't suddenly start spinning uncontrolably.

@XenoMurph: correct. Only that release of stored energy could just about in some rare cases be enough to whip the blades round with enough force to chop a finger off. Hence my advice to use a stick to clear the jam before getting your fingers involved.

Can you hear the sound of my head thumping against the wall? People, I know that of which I speak. If you're using powered machinery and it jams, then it might have energy stored in the mechanism because of that jam, and it might be dangerous. So, if your lawnmower jams, make sure it's definitely OFF, and use a stick to clear the jam. 99% of the time there will be no need for the stick. 1% of the time, you might be surprised. And then there will be that 0.1% of the time when the surprise release of energy could chop a finger off. So use a stick - it's not hard.

More downvotes for sensible safety advice. Look at it like this: are you totally sure you understand absolutely everything about every piece of powered machinery? Are you totally sure of the state that the jammed machinery is in? Are you absolutely sure you know everything and cannot have made a mistake? If your answer to all those questions is "yes", you're deluded. Follow sensible safety precautions. If they're not needed - and often, they're not - you've wasted a trivial amount of time and effort. If they are needed, you've saved yourself a bother.

If an engine turns with the ignition shut off, the only thing that could be turning it are the magnets on the stator, and they would only create a minute rotation with absolutely no force behind the action. There is no "compressed air energy" in the cylinder.

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There can be compressed air energy stored for a short while in a cylinder if a piston engine has jammed, particularly 2 strokes. If it has jammed, I say. There can also be elastic energy stored in the mechanism of any powered machinery which suffers a jam - if it's jammed, I say. Any powered machinery has some potential - note that word "potential" - to be dangerous even when turned off. You have to learn how to handle machines safely. I've suffered several minor injuries from stored elastic energy even when using hand tools, and never mind powered machines. I've learnt caution the hard way, but thankfully without losing any body parts.

(Wow. Downvoted for giving sensible safety advice based on, ooh, too many decades of experience? Including experiencing all sorts of interesting kick-back type events from piston engines - most when the spark's fired on starting and sent the piston back the wrong way, but not exclusively. And never mind the elastic energy which can be stored in almost any powered mechanism that's jammed... Be cautious, people. Shut the machine down fully and clear the jam with a stick. Then you can get your fingers involved.)

I see. So I get downvoted repeatedly for giving good safety advice which is based on: reading the instructions; paying attention to what experienced engineers and technicians have taught me; and personal experience. Those experienced engineers (as in "chartered") and technicians really were *very* experienced, their experiences going back to the 1920s in at least one case. One last time: any powered machinery which jams might have enough energy stored in its mechanism somewhere to cause serious injury. It cannot be considered safe until the jam is cleared according to the instructions. If you don't have the instructions handy, proceed with caution. And use a stick!

Alternatively, you could follow the advice of utter twerps who think that because they've always got away with it so far, then nothing can go wrong. Don't come crying to me when something unexpectedly removes the top joint of your thumb.

FreeTheUnicorn, Secret Squirrel, and Aline below all have very valid points. Read what they have to say, learn from them - please! Powered machinery is dangerous, all of it. You have to follow appropriate safety procedures to keep yourself safe. Their advice is all good - all of it. I used to love Secret Squirrel on the telly, many decades ago.

We get monster cones from our oldest pine. They are as big as small pineapples. The grandkids think they're cool so they collect them before we mow

Some pinecones are really sappy at certain times of the year so they're dense and have some nice resistance to impact with a blade. We had Douglas Fir in our backyard and in the fall one pinecone under the mower meant a ton of time removing the thing and then cleaning the sappy mess before you could get back to mowing.