"There we go," the Master purred, wrapping the memory loop up with that memory, using it as a trigger, a smug, satisfied smile on his face as he buried the loop back into Jack's mind with care, building careful protection around his work.
Simple, but vicious, a psychic switch that would convince Jack's body that it was experiencing a Dalek death ray once more, killing him while anyone trying to root out the memory loop and its trigger was still in his mind. And a nasty surprise for the Doctor if he tried to make love to Jack - the trigger for the memory loop.
Then, carefully, delicately, he pulled back, piecing the bits of Jack's mind he'd ripped into back together, leaving them bruised and battered, but leaving just enough chaos to hide what he'd actually done, as he deliberately raked through Jack's memory of the last few moments, shredding them.