Floor switch?
 

What is the round switch on the floor under the gas pedal?

It's the kickdown switch, which may or may not be enabled.

More info here:
http://forum.a8parts.co.uk/showthread.php?t=50

Kick down I believe.

When the 'gas' pedal' is pushed all the way down and hits the button it drops down a gear

Beat me by 30 secs!!

Not that useful on a manual swap. Use it for nitrous :D Or exhaust cutouts?

Thanks guys. Good thing my car doesn't need it anymore. I had troubles with removing the big plastic base when I had to remove the side transmission tunnel panel to check the orange drains. I accidentally broke the switch and never got around of fixing it as it didn't affect driving the car or turn on any warning lights.

I had no idea there was a switch integrated into the pedal stop...

I reread parts of that other thread and noted the very very simple "check" procedure, and it works! +++
1. Get into car. DO NOT insert key or turn ignition on!
2. Check gear selector is still in P. Turn sidelights on, look at illuminated +/- symbols on gear surround.
3. Press accelerator pedal to the floor, those +/- symbols get brighter (and you should hear the switch click)
Simples! :D

Haha so it does! :D

Very late to this thread. BUT teehee haha. Yes it does work

It's good to repeat this procedure from time to time as it's fairly easy to inadvertently switch the kickdown off (I've done it).
Without it enabled the cars lifeless and flat, and this switch is easily overlooked.