I am not practicing pranayamas yet,but it must be dependent on pranayama you are actually doing. For example ,according to Iyengar, in ujjayi pranayama (legs crossed, with feet on the hips, stretched hands with your palms up on your knees) you hold a breath after full inhalation for 1-2 seconds, than exhale for 2-3 seconds, pause for a second and then inhale again.
Hand manipulation is probably done to force your chest muscles move in right direction.dunno. have to ask gurus
