Chris, IŽll try to keep this simple with a more practical explanation.
First of all, torque is produced by the engine. Torque becomes apparent when the engine is trying to overcome the resistance force(s) acting on the "output end". In general - The higher the resistance, the greater the torque.
Lets use a powerful cordless drill as an example. If you drill into a hard material like concrete or steel, you might have to hold the cordless drill with both hands in order to prevent it from rotating in the opposite direction (in relation to the drill bit). But drilling into styrofoam or wood can easily be controlled by using only one hand.
When the rotorblades on a helicopter are turning, they experience a resistance force called drag. The drag changes for a number of reasons (angle of attack, pitch angle, density of the air, etc.). More drag - the engine has to work harder in order to keep the RPM constant - and a higher torque is the result.
Hope this helps!
rotordude
Last edited by rotordude; 26th Jun 2009 at 01:45.