I'm not with Laminar Research, just a fellow simmer in the UK and apologies if you know this already, but 121.500MHz is a frequency band reserved for emergency communications for civilian aircraft in distress.
To tune into the emergency channel very quickly, the COM1 flip-flop key is pressed and held, so that 121.500MHz is set up automatically in the GNS530 panel.
If you simply wish to switch between the COM1 standby and active frequencies, then the flip-flop key is pressed and released in quick succession.
Bottom line: short press of flip-flop key to change frequencies, long press to select the emergency channel.
The same thing applies for COM2 in the GNS430 unit below
Hope this helps?