I’ve not actually fitted this device but based on reviews it makes the revs drop quicker when releasing the accelerater between gear changes so making better gear changes.
As described by the sellers …… Idle air control IAC restrictor plate, fits for Ford Mustang 86-04 V8 V6 GT 4.6 5.0 3.8, for Ford Mustang LX GT 5.0 V8 86-95, for Ford Mustang GT 4.6 V8 SOHC 96-04, for Ford Mustang Cobra 4.6 V8 DOHC 96-04, for Ford Mustang 3.8 V6 94-03.
Premium quality material, excellent durability and long service life.
Meet the quality standards, simple to install, no modification required.
Perfect to replace old or broken idle air control IAC restrictor plate, good replacement.
-Reduces or eliminates RPM rev hang when shifting, decelerating, coasting etc.
-No check engine light, no tuning or other modifications needed
-Allows faster shifting and saves fuel
The only reviews I found are …… https://mustangforums.com/forum/4-6l-general-discussion/583855-is-this-the-rpm-hang-thing-that-i-can-fix.html
and …… https://www.tccoa.com/threads/i-tried-an-iac-restrictor-plate.137625/