Channard Cenobite: And to think... I hesitated.
Pinhead: It is not hands that summon us. It is desire.
Pinhead: Your suffering will be legendary, even in hell.
Channard Cenobite: I recommend... amputation.
Kyle MacRae: I'll get your clothes. I can do that. I'm a doctor.
Answer: All we can see (in abstract closeup) is serpent-like tendrils cutting him and driving the pins into his head. As someone else said, it's likely another cenobite, although alternately it could also be the same "contraption" that turns Channard into a cenobite late in the film, given the tendrils are similar to the ones he sprouts.