I'm just wondering how much this "regen" actually can effect run time. it cant be much.
Not much no, very small amount, it's not really optimized in these R/C applications vs. real electric cars regen brakes.
I understand that when you spin the magnet inside the coils the motor acts like a generator. But wouldnt it act like a generator while its just coasting too? Why does it take shorting the motor phases to generate energy? Just not making sense to me.
Yes, when you are coasting there is potential between all three phases, however there is no current, hence no power being produced. The way the braking works is basically a 'controlled' short, you spin a motor in your hand, and it spins quite freely, then short out the leads and you have a dead short, that is basically the most braking action you can get from the thing. The FET's just control this by 'pulsing' the shorting action, thus the amount of braking effect.
If you lock up the tires when stopping, then the motor is then no longer spinning so it cant be generating any power to send back to the battery, but it must consume some energy in order to keep the motor from spinning. How can it not take engergy to prevent the motor from moving?
Despite what you may THINK you are seeing, the wheels really aren't locked up, they are spinning a little bit, and that little bit is basically the motor doing 100% braking action under those circumstances. And you are correct in your assumption that the motor cannot be generating potential when it is not moving... it's just that the motor IS moving, just very slowly, it doesn't take much to induce a potential into the windings, and thus currents.
I guess I am still not clear on this.