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.
