[Kevin Sevcik]

So turn the robot on and leave it in disabled mode. Try turning your wheel by hand, then unplug one of the motor wires and try turning it by hand again. If it doesn't get easier to turn when you unplug a wire, then it's definitely in coast.

Also, if it's fairly hard to turn even with a wire unplugged, then you just have a lot of mechanical resistance in your system. That would also explain why you're seeing better stability with a -0.3 instead of 0. The extra mechanical resistance is slowing your wheel down more rapidly, so giving it a little push instead of letting it coast makes it ramp down slower. So you're having to bump it less often to keep it in tolerance, which I assume is what you're counting as better stability.