The over boost protection solenoid performs two valuable functions. It vents excess boost (over 14 to 15 psi) in the event of waste gate malfunction, and it vents unneeded boost during deceleration. This latter function allows the ALDA to cut back on fuel enrichment during off throttle operation resulting in better fuel economy and lower emissions. I posted this like with another thread but it serves well here too.

http://hsb.baylor.edu/html/easley/autofaqs/boostchk.htm

Bob D.