A Hair Comb is a hand movement where the leader moves a hand straight over the head of the follower or leader. Then letting go of the hands gives the opportunity to “brush” or “comb” the hair (you never comb the hair of the other person, only your own hair). Often the leader puts the hands over the head of the follower, giving an opportunity for the follower to style the movement. It can also be done over the head of the leader, which is often used as a purposeful action to change the position. See Cross Body Lead Hair Comb for example.
Note that Hair Combs are mostly done with crossed Hand Holds. This feels the most natural. However, it can be done in any position.
Note that followers can think a turn is initiated if the follower is unfamiliar with a hair comb. It helps to only initiate a hair comb on 1 in a Basic Step, because a follower should never do a turn on count 1 (only on count 5).