The older Ford starter uses an actuator arm for engagement. It works from an electro-magnet pull. His bouncing got the arm to move so that it could engage more easily. Either there is a problem with the magnet or the arm itself. You can get a rebuild kit for the starter and fix it yourself, buy a rebuilt starter or buy a brand new starter. But before all that, take the starter into a parts store that can test starters. They will be able to tell you if it is good. The problem may be a relay that the bouncing jostled and not the starter, or something else.