When Westley Raised his hand in anger at Buttercup

This was a VERY important part of the movie.
He was in "character" of "The dread Pirate Roberts" who was feared by both men and women and he did this action to instill fear into her because needed her to be afraid. His goal the whole time was to make her "Not afraid of the "Dread Pirate Roberts" and not afraid to confess her love for Westley and push the pirate down a steep hill. He wanted to see if the most feared man of the 7 seas would stand in her way of true love and he didn't.