new truck-wiring question
 

I just bought a 97 3/4 ton 4x4 with the 7.3 and after I got home I was going over everything again and found some wiring alterations done by a previous owner. On the passenger side of the firewall in the engine compartment there is a module with 3 or 4 wires going to it and a line from the intake manifold which I'm guessing gives the module an input for boost level. Two of the wires going to the module were tapped into and connected to a switch. My questions are what is this module and what happens when you close the switch, connecting those two wires?

map sensor