There are two times when you can get out of your hog to defend yourself.
When any enemy hog that is following you starts shooting you.
When an enemy rocket hog with a gunner is coming towards you.
In both cases, the enemy has to be close enough to be doing damage to you and you would be able to throw a nade on them with a reasonable throw (they should be on your radar scope).
From - Joxer on old forums 7-13-2006
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