Lencodrive at the finish line

a friend of mine tested his 2 speed lencodrive for the first time this weekend. Everything was great with no problems besides a very hard engine brake when he let off the pedal at the finish line , the engine brake was so hard the rear tires started to skid and the car jumped all over the place and touched the guard rail slightly. There was some talk about the converter causing this but it seems to work fine when stalling and during the run. If the engine was shut off and pump in transmission didn't turn would it still be the same or could he just let of the pedal and shift into 1st gear at the finish line?