How do "tip jet" helicopters cancel the torque effect of the main rotor?
This is a basic physics question, involving Newton's third law of motion  (For every action, there is an equal and opposite reaction.) When a centrally mounted engine applies force to turn the rotor, the equal and opposite reaction creates torque on the fuselage.  With a tip jet, the force is applied by the jet shooting its exhaust perpendicular to the blade, so the reaction is that the blade moves in the opposite direction.

So the answer is that tip jets don't CANCEL the torque, they simply don't create the torque in the first place.