Unstacking is nothing more than the company tagging junior in category to cover flying over unpopular days (3 day weekends, holidays, etc). If you have PBS and are junior in category you will cover all the days that senior people don't want to fly, before any of your bid days off is considered.

Compare Xmas schedule for example, my golf buddy is SWA with line bidding. Xmas falls on a wednesday for example, in line bidding senior line holders have to bid a line that works weekends to get xmas off (disregarding vacation over holiday holders).
In PBS, if you are senior, you can manipulate your schedule during the bid to have weekends and xmas off, and the junior folk will get "unstacked" into xmas and any low coverage weekends to make up the slack.
PBS really magnifies the power of seniority.
Everyone may enjoy the occasional ability to avoid an important date in PBS, whereas in line bidding you are already making concessions to get what you need on the outset.
Either way it's good to be senior, but in PBS it's really good to be senior.
YMMV
LUV