A continuous variable transmission in lego, this is an automatic transmission which uses no motor or other mechanism to change gear ratio, but changes this automatically by the gears themselves.
Forward: automatically switches continuously the gear ratio from 1:1 up to 5:1
Reverse: gear ratio exactly 5:1
2 Power XL motors, one for the left side, one for the right side.
The gear ratio is changed automatically by the gears themselves by detecting the power which they exert on their surroundings, which happens separately for the left and the right side. The force at which this happens can be controlled by a lego spring.
This mechanism normally uses 2 differentials per motor, but then when the wheeles are blocked while in reverse, the motor still keeps spinning, and the maximum torque on the wheels is equal to the torque caused by friction inside the differentials.
This is no good, but by adding a third differential this problem is solved. Now in reverse the gears act just the same as a regular 5:1 gear set. The total number of differentials in this application is therefore 6. You need 3 differentials for each motor.
This robot drives as if it doesn't have the huge gear set! There are only 4 transitions where torque goes from one gear to another between input and output.
The clicking sound is caused by an oscillating part in the gears, which is perfectly normal ;-) it isn't a bad gear connection or something breaking down. It is caused by the robot lying upside down, otherwise gravity prevents it from oscillating.
A composite picture of an older version, illustrating the basis of the mechanism: http://img843.imageshack.us/img843/729/compolego.jpg
This is an illustration of another mechanism, based on the same principle: http://www.brickshelf.com/gallery/miezel/Other-Stuff/AUTOTRANNY/0.png