I suppose this sort of thing doesn't break the game, but in the scheme of things, it doesn't make a whole lot of sense. Why couldn't you just press and hold the attack button while in the air to do this? As it is, you can't do these things unless you're coming out of a transformation. If you hold the melee attack button instead, you'll do a ground stomp. If you hold the fire button to transform, and then hold the jump button, and then release the transformation button, you'll jump as you transform. There are a number of moves you can do while shape-changing. Also, your firing button then changes to your melee attack button, despite the fact that you're firing guns or whatever. This is fine except that you can't not accelerate while in your vehicle form.
In order to let up off the accelerator, you need to let up off the trigger a bit (if you're playing with a controller with analog buttons). Once you transform into your vehicle mode, you have to keep holding down the standard fire button.
