The lights (combustion of the fuel) happen at the top of every other movement of each piston. If you watch just the left-most piston, you’ll see the 4-stroke cycle (aka Otto/Diesel cycle, aka suck-squeeze-bang-blow.)

If you start watching the first piston right as the light flashes, the piston is at the top of the stroke length. This is the combustion portion of the cycle, or the “bang” part. This combustion creates rapidly expanding gas, pushing the piston down the cylinder.

Once it gets to the bottom of its travel, you’ll notice one of the valves at the top of the cylinder open up. Then the piston rises back up the length of the cylinder. This is pushing the exhaust gas out of the cylinder - the “blow” part.

After that, the exhaust valve closes, and the intake valve opens up. The piston is traveling back down the cylinder, due to the other pistons rotating the crankshaft. This pulls in air, and at the same time, modern internal combustion engines will spray the fuel of choice into the cylinder via fuel injectors. This is the “suck” part of the cycle.

Next up, at the bottom of the travel again, the intake valve closes. The piston travels back up the cylinder. Since it is now a sealed environment, the air-fuel mixture inside is compressed. This is the “squeeze” part. Then we’re back to the “bang” when the light goes off again.

In a diesel engine, the pressure and temperature alone cause the fuel to ignite, while gasoline engines utilize a spark plug for ignition. Ignition from pressure and temperature alone in a gasoline engine is referred to as detonation, and is a particularly bad thing to have happen.

It’s a little unorthodox to start the explanation of the power cycle at the ignition phase, but I think it’s useful to have the light as a good starting point when there’s a visual representation.

Hope this cleared something up for you!