PBS can best be explained to the XJT guys as line bidding each week. Trips can start on any day of the week and the more junior you are the more likely will be flying over the weekend and more days in a row.
Say you like mon-wed 3 day trips. You can sort those out so you only look at those trips for that week then you put those trips in order of preference.
Now go to the next week and do the same until you have bid for all the weeks in a month.

Now say you really need a Monday off. Well in hard lines you might get lucky if there is actually a line that has what you need. You are more than likely going to have to swap. Not so with pbs. Assuming you can hold mon-wed, you could probably hold tue-thur. So that week you select tue-thur 3 day trips and you get what you want the first time. No need to jack around with open time and waste time on the computer.

You have to be realistic in what you can hold. A junior line holder now that holds 4 days fri-mon isn't going to magically get weekends off, but there is a lot more flexibility if he needed a Friday off.