Crouch - details and controls
The Crouch control is on the left side of the Secondary Screen, second button from the bottom.
Each 'Mech is capable of kneeling in order to make for a shorter target, hide behind a barrier, etc.
To make your 'Mech crouch, you must first bring your 'Mech to a complete stop, then hit the Crouch Button.
To start moving again, hit the Crouch button and the 'Mech will stand up.