This device is designed to work with your 2 1/2" shop vac hose.  The large end is for attaching a piece of flexible tubing, and the small end is for attaching your shop vac.  I have it attached to my Rigid 4512 tablesaw: flex tube from the saw port to the large end of the reducer, then attach the shop vac to the small end.  Secured with worm clamps.  Works like it's supposed to for me.