Rules for this type of upgrade instruments are similar over all airlines.
- You need to book a ticket in a minimum fare class. In Alaska's case that's Y, B, H, K, M, L, V, S, or N. Typically, cheap tickets don't qualify
- There needs to "upgradable space", i.e. tickets need to be available in the fare class "U"
- If there is no "upgradable space", you either can't use the instrument or are placed on a waitlist with complicated rules.
- Hit rate is generally very low (unless you are top tier and/or lucky) and may require significant research and flexibility.
The idea is to claim that they give you "free upgrades" but make it so restrictive, that most can't be used, so it's mostly free for the airline. For example United offers free domestic upgrades even for the lowest tier, but that just means they make the wait list longer. I have seen upgrade waitlists with more than 130 names for one open seat.
Rule #2 is the main killer. It's entirely in the discretion of the airline when and how to release upgradable space and they rarely do. Your best shot is to get waitlisted (if Alaska does that) and hope for the best.
If you want to play that game, you need to monitor available fare classes for the routes and dates that you are interested in. There are 3rd party tools like https://www.expertflyer.com/ (no affiliation), that can help with that but it sure is tedious and it does require flexibility. I used to put work into it but even with flexibility and and a fair bit of experience a good chunk of my upgrade instruments expired unused. IMO the airlines have devalued their loyalty programs to the point of becoming useless. These days I just go for the lowest bidder for what I want.
You may get some real world tips and tricks for Alaska airlines at https://www.flyertalk.com/forum/alaska-airlines-mileage-plan-442/
The rules for Alaska are here: https://www.alaskaair.com/content/mileage-plan/membership-benefits/guest-upgrades