If you use a second floppy drive connected to your Amiga 500, like GOTEK drive or a mechanical floppy drive, (as DF:1), then you need a boot selector to boot from the second drive. (Most of old boot selectors work with a toggle switch.)
While using "BOOT DRIVE SELECTOR", if you want to toggle your floppy drives (DF:0 <> DF:1) you just need to press Reset keys 3 times, or instead of 3 times, you can press continuolusly reset keys for about 4 second. (Reset Keys are: CTRL and the two Amiga keys together) . 30 seconds after last reset, counter sets itself to zero.
Benefits of a switchless Boot Drive Selector:
1) This Boot Selector protects your Amiga 500. If you change the mechanical switch position while transfering data from floppy drive, this may cause damage to 8520 (Even CIA) or 5719 (GARY). Switchless booter do not let this happen.
2) Most users need to make a hole on their Amiga 500 for attaching boot selectors mechanical switch. This "A500 - Switchless Booter" toggles drives elecronicaly, there is not any mechanical switch, so you do not need to make a hole on your your Amiga.