Let’s say there are 10 rooms in a category. If on the first day of the 11 month window, you book that day plus the next five, then you’re holding one of the 10 rooms for at least those next 6 days.
That room would not show up in each of the next five mornings as an available room. If eight people were walking already, at 8 am the first two people could grab the remaining two rooms and then there’d be no more availability. If 10 people are already walking all 10 rooms, there’d be no availability even before the 11 month window opens.
So. 4 days later, you modify your reservation to add 4 nights to the end and remove four from the front of the reservation, and you’ve walked the reservation forward by 4 nights.
In the meantime, nobody could book the room out from under you because for at least that room, you are blocking it’s availabiity at the 11 month window; you’re occupying that room already on each morning the 11 month window opens.
Once you secure the room at the 11 month window, you are blocking anyone else from it for as long as you hold it with days beyond the 11 month window, days that you keep modifying forward until you get where you want to go.