When the main track is knocked down, the tank will continue to move, thanks to the reserve track—but its speed will be reduced. If you lose another track, its speed will be reduced further. But the tank will still be able to turn and move forward and backward.
The M-V-Y travels at a maximum speed of 40 km/h. If you hit the front roller and knock down the main track, the tank will slow down by 40%. If both main tracks are hit, its speed will drop by 65% of its maximum speed, but it can still move forward/backward or turn the hull to block damage.