The Comb's power was to stop time, but only for about ten seconds when run through the user's hair. The effect of the user moving during the timestop induces motion sickness and even vomiting if used too many times in quick succession. To counter this, Harold often carried around motion sickness pills. The user of the Comb is unable to affect or move anything during a timestop and if not perfectly still when time resumes may lose their footing and stumble or fall. Harold advised that all one could do is run and hide, run and hide.
The Order came dangerously close to getting the Comb, and only failed due to the intervention of Joe Miller and Jennifer Bloom, and the Comb came into possession of the Legion. However, Jennifer later got the Comb back to use in Room 9, along with the Key and the Watchbox. It then presumably remained with Jennifer as the Legion (or at least a part of it) was annihilated by Karl Kreutzfeld.