Not the door. There's an arrow pointing to it on the Calibrate page in the manual. The text says, "Turn the rotary disk to the position C, and trigger the measurement by pushing the measurement button." I assume they're referring to the half-moon shaped disk with the three dots on it. On my device, it appears to be depressed on one side but not the other.
Also, on the pages illustrating how to scan a print, the picture shows it being pushed by the thumb.