Three different viable methods of kinetic energy storage device tried to date seem to be:

1) Pin/cam engaged flywheel as on the Whyachi machine and one of 'Dale's robots' creations
2) Screw thread/momentum based 'dog clutch' trigger, as a builder called Glen in the US has tried and me at beetleweight scale
3) Continually rotating 'flipper' with spring-steel or similar contact surface and and high-speed flywheel maintaining momentum through an impact, as on this machine

Never come across a conventional brake based flipper, not that I can remember anyway

Actually I'm looking forward to having some spare time to finish that beetle.. one day