I wondered too. I found this:
COASTING ENRICHER - Feeding a Closed Mouth
You're blasting down the trail and you see a hairpin coming up, so you close the throttle. This closes the butterfly valve. You and your 400lb baby (the bike, not the other love of your life), are still rolling with inertia, keeping the engine revs high. Now it can't get the mixture it's trying to suck in because the butterfly valve is closed, blocking the air flow. To compensate for this there's an air jet in the lower diaphragm chamber that transfers ambient pressure to one side of the coasting enricher's spring loaded cut off valve. The excess vacuum in front of the butterfly valve is transferred to a drilling that leads to the other side of the cut off valve. This sucks it open, allowing the pilot jet to feed more fuel to the engine preventing an overly lean condition. (Pop. Pop. Pop.)
This isn't a TK carb, but the principle is the same.