The II Plus had the so-called "Autostart ROMs", meaning that it will attempt to boot from disk on power-up. If no system disk is present, Drive 0 will spin endlessly until the user presses Reset (or Ctrl+Reset on machines with the Ctrl+Reset safety switch enabled) to enter Applesoft BASIC.
The II Plus's "Autostart ROMs" were a feature that automatically attempted to boot from a disk when powered on. If no system disk was found, Drive 0 would spin endlessly until the user intervened by pressing Reset (or Ctrl+Reset). This allowed users to access Applesoft BASIC.