When you jack off for a really long time, and some of the cum that's will eventually burst out just sorta sits on the tip of your dick. Prebust is normally clear, not white.
Jack: Dude, I have sooooooo much prebust on my tip!
John: Well you wouldn't if you weren't jacking off for so long!
Jack: But it's the Animal Channel. It's the only porn I'm allowed to watch!
When you have a lot of prebust
on your tip, and then you finally burst, making it look like a great big blob of white rising from your sea of clear prebust. Just like a great white shark.
My Great White Shark flew all over her face.
It took forever, but when I let out my Great White Shark, it landed right on the tip of my toenail!