12V 100RPM High Torque Worm Gearmotor

In sumo robot or combat robot competition where high holding torque motor is essential to avoid your robot being pushed by your opponent robot. The worm gear is a better option compared to the spur gear motor. 

"Unlike with ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible when using large reduction ratios, due to the greater friction involved between the worm and worm-wheel, when usually a single start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input."

In other words, it is difficult to push your load (for example, your robot) if the motor is not powered or rotated.