Additionally the glutes are activated and when that happens the lumbar spine turns off as much as possible. The abs are definitely, definitely, definitely firing not only since the back is turned off, but because of the movement itself. No matter how you move, with or without weight, the core muscles(all 26 of them) are the first to fire.
The movement of Long Cycle(kettlebell clean and jerk) is doing many things at once. From the very first movement, the clean, you pull the bell(s) up to rack position using the posterior chain. Next it is driven overhead in an explosive manner using many pressing muscles and pulling muscles on the ascend and descend of the bell(s). Then conditioning is effected cardiovascularly as well. So both anaerobic and aerobic thresholds are pushed in a the direction very few other movements, exercises, and lift can match. A top strength and conditioning coach once was asked if there was one exercise that he would ever do and/or recommend doing for the rest of his life which would it be. He said the single arm(dumbbell, kettlebell, etc.) clean and jerk. That would be Long Cycle.
But don't take it from me. View it with those who are much more experienced in GS than I am.
Valery Fedorenko, who is an Honored Master of Sport(which basically means he's the Tiger Woods and Michael Jordan of Girevoy[Kettlebell] Sport), displays the absolute perfect form for the rack position in his various sets displayed in the following videos. Featured in the last video is the current best in the world, Ivan Denisov, as well as Andrew Durniat, Ken Blackburn, Ginko Vasilis, and Roberts Innus.
1. Valery Fedorenko performs a 10 min. Jerks Set Using Two 70lb. Kettlebells and achieving 130 reps to earn Master of Sport:
2. Who is Valery Fedorenko:
3. Valery performs 3 minute Long Cycle with two 53lb. Kettlebells. He does this very fast, but you can still see his form:
4. A video analysis of GS champ Valery Fedorenko performing a GS Jerk with two 24 kg bells. The curves on the right are "force to the vertical direction" and "momentum of the weights". The maximum force applied is 1200 N.
5. World Kettlebell Lifting Championships - Kettlebells - Jerk: