An employee was performing maintenance functions on board an aircraft. The employee manually reclined a passenger seat back and raised the foot rest to inspect under the seat. Using his right hand, the employee pulled on the manual release mechanism to lower the foot rest, not realizing that his left thumb was between the foot rest support structure. As the foot rest was released, it automatically dropped to the folded position, severing off the employee's left thumb at the first knuckle (approximately 1").