For those that don’t know ascii off the top of the head, it prints either a forward or backward slash repeatedly, which in the c64 font creates a pretty art piece like the graphic at the bottom of the screen.
ASCII / (0x2f) and \ (0x5c) has three problems:
1. They're not as conveniently located (chr$(47+(rnd(1)*45) isn't quite as pretty)
2. They don't fill up character cell so there are gaps in the "maze"
3. PETSCII doesn't have a \ (0x5c is £)