As Chris pries with his knife, long blades hidden under the lip of the coffin suddenly swing out in an arc -- much like a needle on an odometer. Although the Mekhet isn't entirely caught off guard*, one connects as he is withdrawing his hand.
The blade is razor sharp, slicing cleanly through his hand at mid palm, littering the coffin with his fingers.
Someone slower would most likely have lost their hand at the wrist; someone still alive would most likely be bleeding to death right now.
The blades silently disappear under the lip again, their damage done, and the trap presumably reset.
Chris Wooding takes 2 Lethal, and loses his hand below the thumb.
* 1 PER failed, 1 PER passed