[Archon_Wing]

My theory in DBZ is that Goku is frequently seen eating in the anime, and thus he suffers from really bad constipation and must go super saiyan to go to the bathroom

Anyhow, it's a way of suggesting the character is in deep pain trying to bring out their ultimate limits. You want to be so intense that every part of you is being pushed. Unfortunately, it tends to go on for too long.