HI,
Yes, it will start the RBT decrementing once the NAV timer reaches Zero.
here is how it works,
1. When a client wins the medium, it will send RTS with DID (Duration id, Approx time it want to hold the medium)
2. All other clients those are in the same BSA will set their NAV time equal to DID and decrement by one for every slot time. (Which is called the Virtual carrier sensing )
3. Once the NAV timer becomes Zero, All clients will do CCA (Physical carrier sensing) means will sens whether any frames are going or coming from the same BSSID where they are connected with.
4. If CCA is clear then they will decrement RBT by one and again all clients will do the step 3 (CCA). if CCA is clear then they will decrement the RBT by one and continues till the RBT=0
5. Once the RBT becomes Zero (Any one client's RBT will becomes Zero) and the CCA is clear then that particular client will send RTS with the DID .
6. Once RTS is seen in the air and the CCA will fail then other clients will pause their RBT and will set their NAV with the new DID.
the above cycle will repeat.
hope you got some clarity on this.
For your ref :