You need to use something along the lines of a 965 BOV that can be welded into your CIS/turbo plumbing.
When the throttle is chopped the exhaust gasses slow the turbo. The diminishing boost on the pressure side bi-passes the throttle plate when the BOV opens. This same boosted air flows right back into the turbo inlet and continues to cycle across the turbo as the engine slows down. The result is that you are using every bit of intake air energy available to keep the turbine spinning. The decelerating exhaust is what slows the turbo at that point.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8