FLEXPLATE TO FLYWHEEL CONVERSION

He is possibly taking about a coupler clutch ?
With the clutch pedal released it is locked with splines,
pedal 1/2 way down is disconnected, all the way to the floor engages a cone clutch to get the car rolling then let out the clutch to lock it solid. short track stock cars use them. Mostly chevy and it bolts to the crank with the flex plate flywheel.

Southwest Speed - Performance Auto Parts