Energy from flywheel rotation is lost via the wheels when sitting still. It's transfered to the chassis, which bleeds it off into the wheels and into the rider. Proof of that is simply watching the handlebars shake.
Hold the brakes on, and the bike is forced to be stationary, and that little bit of energy that would have made the bike move a little bit, is now kept in the chassis, and forced to show up somewhere else.
That energy will translate, like electricity, to the easiest path: The rider on the bike.
Don't forget, your flywheels spin forward, (towards the front of the bike) and as each cylinder fires at 315* with a 405* gap in between.
“When the going gets weird, the weird turn professional....” ~ Hunter S. Thompson~
R.I.P. Rubberdown. 1971-2012