Heavy Traction Mode is a secondary drive mode that was only in Battalion Wars. In all ground vehicles hold the B button to engage Heavy Traction mode. This was probably added due to the vehicles sometimes getting stuck because of the games sometimes bouncy physics. The mode allows the players vehicles to free themselfs if stuck. This was introduced during Assault on Windbreak Ridge by Colonel Austin.
This feature is removed in Battalion Wars 2 probably because it wasn't needed because of the games better physics.