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.
Co-worker: "Have you
seen the bosses new secretary? She's fucking hot."
Me: "Yeah. When bossman went to introduce us I got so
hard I had to perform the one-hand
tuck."