For those not in the know, a blow-off valve is used on a turbo to vent the boost pressure generated by the turbine to the atmosphere, when the throttle is closed. This will help with two things. Firstly it prevents the turbo from stalling because it would eventually hit a wall of compressed air in case the throttle is closed for a pro-longed period of time. Secondly it also helps with the turbo spool up after you open the throttle again, because the turbo is still spinning at a higher rpm and therefore will have to accelerate less and spool up quicker. Of course this is only possible, when you run a blow-through turbo setup.
So here goes: