What is a pump-up?

In one line

A pump-up is light resistance exercise done just before going on stage to send blood and fluid into the muscles, making them look bigger for a short time.

What is a pump-up?
Why work out backstage right before a contest?
Backstage at bodybuilding contests, some competitors keep lifting barbells and dumbbells until just before they walk on stage. This is called a 'pump-up'. This post explains why people exercise right before a contest.
In one line
Exercise that briefly swells the muscles just before stage
A pump-up is light resistance exercise done just before going on stage to swell the muscles for a short time so they look bigger. The word comes from the English 'pump up'. It is one part of getting ready backstage on contest day.
Background - the pump
Muscles swell for a while after exercise
When you do resistance exercise, a lot of blood rushes into the working muscles and the water inside the muscles also increases. So right after exercise, the muscles swell and get thicker for a while. In the gym, this is called 'the pump'.
Pump vs. pump-up
A thing that happens, and making it happen on purpose
The pump is something that happens on its own when you exercise. A pump-up is making that pump happen on purpose, timed for when you will be on stage. It uses the same effect, but 'pump-up' is the word used backstage on contest day.
Where it happens
The waiting area next to the stage
A contest venue has a waiting area where competitors gather before going on stage. The IFBB General Rules require the organizer to place barbells and dumbbells of various weights and benches in this area next to the stage so competitors can warm up. Competitors do their pump-up here.
When and how
Right before going on stage
Competitors pump up right before going on stage, using resistance exercise with light weights and many repetitions. The key is timing it to when they will be on stage.
Why do it
To look bigger, and to balance the body
A pump-up makes the muscles thicker for a short time, so they look bigger on stage. Competitors also pick body parts that look less developed and pump those up, to even out the balance of muscles across the whole body.
It does not last
That is why it is done right before stage
Once you stop exercising, the swollen muscles soon shrink back. Thigh muscle that got thicker from one set of exercise went back to its original thickness after about 30 minutes. That is why a pump-up is not done early, but right before stepping on stage.
Common misconception
Is muscle swollen by a pump-up new muscle?
No. What got thicker is muscle swollen by blood and water rushing in for a while, and it soon shrinks back once you stop exercising. Real muscle growth is a change that builds up little by little over weeks of continued training.
Summary
Briefly swelling the muscles just before stage
A pump-up is light resistance exercise done just before going on stage to send blood and water into the muscles and make them look bigger for a short time. The swollen muscle is not new muscle, but a short-lived change that soon goes away. Related terms: pump, peak week, carb loading.