Trivia: The horn sound that is continually made by Leviathan is Morse code for "God".
Trivia: If you pay attention, the Cenobite "Chatterer" first appears eyeless, then has eyes later in the film. This is because the actor underneath the makeup (Nicholas Vince) hated performing blind and requested the makeup be redesigned to give the character eyes. A scene was filmed that showed Pinhead giving Chatterer eyes as a sort-of gift, but the scene was cut, leading to the continuity error.
Trivia: Dr. Channard's name in the script was Dr. Malahide. "Channard" is derived from Christiaan Bernard, who performed the world's first successful heart transplant.
Trivia: As originally intended, Julia was meant to become the new primary antagonist of the series after this film, with Pinhead and the other cenobites remaining dead. However, the Pinhead character became incredibly popular, and Julia's actress Clare Higgins decided she didn't want to do another sequel. So Pinhead was resurrected in the following movie and became the central figure of the series.
Trivia: The film was originally meant to co-star Andrew Robinson as Larry Cotton, who would be found in hell and would accompany Kirsty and Tiffany throughout the remainder of the movie. However, Robinson turned down the chance to reprise his role for unknown reasons. This forced last-minute script rewrites as a great deal of the film had to be restructured due to his absence.
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.