Suppose there are 3 value rooms. Today, you book one for 4 nights as soon as the 11 month window opens. Member A books the second for 7 nights and Member B books the last room for 3 nights.
The next day no one else can book a value room because the first day of their stay is already "sold out". You can't book days past the window if the first night is not available. So the first opportunity anyone would have to book a value studio would be when the 11 month window opens for the 4th night from today - when Member B checks out.
But MS allows those who want to extend their stays to call before the window opens for everyone else. You & Members A & B are virtually guaranteed to be able to extend your stay as long as you call MS before the window opens for the night(s) you want to add. That process effectively blocks anyone else who wants to begin a stay during that time period. Note that you can't add a night that is more than 7 away from the day you call.
So if Member B calls tomorrow, she could add as many as 5 nights to the initial 3 night stay - and since no one else can call or book until the window opens for the 4th night of her stay, she is virtually guaranteed to get the extra nights. Tomorrow, you could add as many as 4 nights and Member A could add 1 night.
Walking is dropping the first night(s) before adding the end night(s) - so that you can reuse the points.
As long as you call to drop and add night(s) before the window opens for the night(s) you want to add, you'll be able to "walk" your reservation until you get exactly what you want. Again, the TRICK, is getting the first reservation. Sometimes, that is not easy.
It's been reported that we can now "walk" online using the modify feature. I am not yet 100% confident that it works, though. If I had to walk, I'd probably call right now. YMMV.