Hans, whilst Grimacher has it's use correct for when I'm going through customs (They never find anything in the back there... it is in fact:
My talent boosters!
You know when guitarists tread on something on stage? well, they're treading on these.. I put mine in a wooden box with a power supply because buying 6 x 9v batteries every 10 minutes isn't my idea of fun! (oh and I'm geeky)
Rarely use them at the same time 23r0, I tend to have one set for generic "Cover up how bad this solo is" mode and the other at the same tempo as a specific song where I use it to build up a wall of cascading notes in time with the tune.
Back in the day when we had an actual breathing drummer I would spend as much time on my knees fiddling with controlled echo feedback as I did standing up with the guitar.. but now we're all electonic and I spend more time worrying about the sequencers..
It actually has US power! :- ) .. the idea is to put the front of your foot on the upper bit, and change the pedals with your heel.. faux foot on a monitor stylee.!