I tried searching before asking, and I know I read it this morning somewhere, but a quick explanation is appreciated.

Thanks!

A blowoff valve is used for force induced cars.. Like turbo or supercharged cars. The blowoff valve gives the air somewhere to go when you let off the gas. That prevents the air from crashing into the throttle plate and backing up into the compressor of the supercharger or the turbo. This prolongs turbo/supercharger life.. and it keeps the turbo or supercharger spooled.

ZuM

At this site...
http://www.se-r.net/about/nx2000/tur.../blow-off.html you will find a photograph and this explanation:

A GReddy blow-off valve releases pressure between shifts and when acceleration is cut short. When the throttle plate is closed pressurized air back-tracks from the plate and heads toward the compressor wheel in the turbo. If the charge air were to impact the blades of the compressor (which are spinning in the opposite direction) undue wear of the thrust bearings in the turbo can be experienced. The blow-off valve avoids the situation by venting the air prior to impact.