system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do some car engines move a little to the side when you press the gas pedal?
gpt
Some of these answers come close to correct, but not quite. 

So- its not 'angular momentum', nor is it 'torque steer' (close). 

It is simply, torque. 

The motor in the car is turning, in one direction, with great force. The motor is mounted to the frame of the car in such a way that the mounts are able to absorb some of the vibrations in the motor (so you don't rattle your teeth out at a stop light). However, this means that the motor mounts will compress/expand very slightly as the motor is 'revved' up, causing the 'rocking' motion that you see in the engine bay. 

This is normal, and is not related to faulty mounts.