Backcombing
Combing hair from the ends towards the roots to tangle it into a cushion that adds height.
Also: teasing, ratting
Backcombing, or teasing, means holding a section of hair up and pushing a comb down from the ends towards the scalp, so the short hairs knot into a soft pad at the root. The smooth top layer is then brushed over it. It builds the height of bouffants, beehives, quiffs and many bridal updos. It is undone gently with conditioner and a wide-toothed comb from the ends upwards, because pulling it out causes breakage.