Loosely grab the dominant "five hand" thumb with the base hand.
Then move the dominant hand downward, representing...er...well you get the
picture.
The base hand stays (for the most part) stationary. The movement is all in
the dominant hand.
Some people misuse this sign to mean
the swear word "sh_t."
But honestly folks this sign is the same sign I used with my kids when they were
little to ask if they needed to "go poop."
The above sign isn't a swear word. The swear word "sh_t" has a completely
different movement, starting position, and ending position. Plus the swear word
version of the sign uses an angry or disgusted facial expression.