The front derailleur mechanism is less complex as shown below. Plates move the chain sideways to transfer it from one chainwheel to the other. The chain tension is adjusted by movement in the rear derailleur. The cage is only in contact with the chain during gear changing and friction losses are unimportant. The sides of the cage must be parallel to the chain to effect a clean gear change.

From: Apps, "The Bicycle Book," Smithmark (1993)