Similar to the penis tuck. The swift tuck you do with your left (or
non-dominant hand, leaving dominant hand to shake hands) when you're in public, standing up and have a visible erection.
Typically, thumb (under clothes) at the base of the erect
penis and fingers at the head of the
penis (over clothes). Then, push wrist down and fingers up. This results in the
penis being tucked under the elastic strap of briefs or any such equivalent.
When done in
one swift movement (that takes years to master) it appears that you are simply adjusting your pants/getting comfortable/scratching. This can be embarrassing but is better than the visibly erect alternative.