Reducing porosity... Whats a surge trap?

I recently saw a video of a pour against a glass plate. That video is featured within this one. It is a very graphic depiction of what a surge trap is used for.

If you're not interested in learning anything skip to 13:37 and watch 12 seconds of me pouring metal out of a crucible.

Andrew Martin's channel https://www.youtube.com/channel/UCFpsAJppHZPEiPQruNfhGgA

Andrew's video https://youtu.be/j2_-pTx4AVg

Видео Reducing porosity... Whats a surge trap? канала swdweeb