Since Blizzard fucked up, there are many weapon procs that can not happen if you have a global cd going when the autohit hits.
Blue post 1: https://us.forums.blizzard.com/en/wow/t/proc-weapons-and-the-gcd/592642/19
Blue post 2: https://us.forums.blizzard.com/en/wow/t/hamstring-reduces-the-proc-rate-of-nightfall-in-2020/598570/12
This wa gives swingtimers with highlighted areas when you can use an ability without blocking the weapon proc, for main- and/or offhand.
It is probably most interesting for people with nightfall or if you want to optimize your frostprocs for viscidus.