[C] Cutting Turn: Flip a coin. If heads, put damage counters on the Defending Pokemon until it is 10 HP away from being Knocked Out. If you do, shuffle Gliscor and all cards attached to it into your deck.
The bold. Since it says "If you do" then that means if you put damage counters on the Defending Pokemon. If you flip tails, you don't put any damage counters, so you don't return to your deck.
Unless this is the Japanese text, though. I could be wrong if it is.