To use this adaptor, take off your transition duct, and then hose clamp it to the 4" part of the Y. Then slide your 3" gray ROVAC hose onto the 3" connector. The brush and rods can then be pushed right through the slit in the seal of the upper Y branch.
WARNING: Carcinogens: This product can expose you to chemicals including Lead, which is [are] known to the State of California to cause cancer. For more information go to www.P65Warnings.ca.gov.