Rek'sai's heavily auto-attack based damage pattern and ability to auto-reset with [Q] makes her a great user of Press the Attack.
Proccing the exposure damage early in a gank will empower you and your laner's damage for a much faster and easier kill.
Triumph allows you to dive deep into the backline with an [R] and get out safely if you execute them, thanks to the massive heal from Triumph.
More Attack Speed on Rek'Sai mean a faster Press the Attack proc and a much faster/healthier jungle clear.
WhyCoup De Grace
Coup De Grace is a well-rounded reliable damage rune in this slot and synergizes insanely well with the execution of Rek'Sai's [R].
The bonus AD from Absolute Focus is fantastic for empowering your ability to quickly clear the jungle. You should be able to keep the buff active for a large portion of your clear with proper healing from [W] burrow.
Waterwalking offers every jungle immense neutral objective power, especially when fighting for scuttle crabs.