it could be a scramble button. some boost controllers, like the Apexi AVC-R i used to have, have an optional wire that allows you to temporarily raise the boost to a higher setting than normal (a bit like when Michael Knight presses his boost button ) but, i'm pretty sure that the button was just a single wire going into the AVC-R and didn't have an earth too
1) have you got a boost controller?
2) does the boost stay at this higher setting permanently until the engine is switched off or does it only last a certain amount of time, i.e. 30 secs
3) is it just the boost that is raised when the button is pressed, i.e. can you measure the differences on your boost gauge with & without pressing the button?