Butterfly hand force curves show two major technique limitations.
1. There is about 2 tenths of a second at the beginning of the stroke where the swimmer is only generating about 5 lbs. The wasted motion is probably due to an arm entry above the shoulders, or beginning the pull with straight arms, or both.
2. There is about 4 tenths of a second where the swimmer is generating about 40 lbs and not increasing the force. The swimmer is probably moving his hands more upward instead of more backward.
The swimmer has a very positive technique element where the force increases from 5 lbs to 40 lbs in about 1 tenth of a second. The swimmer is probably flexing his elbows.