In Unix terms PR# redirects the standard output to the hardware in the indicated slot and IN# redirects the standard input. From then on the BASIC commands PRIN

In Unix terms PR# redirects the standard output to the hardware in the indicated slot and IN# redirects the standard input. From then on the BASIC commands PRINT and INPUT will access that hardware instead of the screen and keyboard respectively. In practice the hardware can actually do anything it wants when you PR#, like booting from an attached floppy.