phosphor() is a tube's phosphor, which kept glowing a moment after the beam had gone: bright things moving fast leave a short trail behind them. persistence goes from 0 (none) to just under 1 (a long trail).
It reads what it showed last frame and keeps whichever is brighter, the new picture or the old one dimmed by persistence. Remembering a frame costs two pictures the size of the canvas while it runs, which is why only effects that need it ask for it.
It goes before crt(), so the trail is of the picture and not of the lines and the mask. ↑ ↓ for the persistence, T to put a tube after it and Space to switch it off.