Originally Posted by dsaul

The front derailleur is not supposed to be shifted under load. Proper shifting technique for the front is to let up on your pedaling pressure until the shift is completed. This is due to the tension on the top side of the chain when load is applied to the system. In order to shift, the chain has to be pushed sideways and catch on the shift pins. The shift pins then lift the chain up and onto the big chainring. If you are applying a load to the chain, it will slip off the shift pins and drop back onto the smaller chainring.