When you enter brush, a poro appears. It will stay behind to give you vision.
Lethal Tempo on Tryndamere allows you to get an insane amount of Auto Attacks in during trades, all-ins, and during your [R]. In the mid to late game your Attack Speed gets so high that you become an incredibly scary threat to the backline.
Overheal synergizes well with Tryndamere's constant self healing from [Q] and BotRK allowing you to enter trades/fights with the early health advantage.
More Attack Speed means more raw damage, kill threat, and split-pushing tower potential.
Last Stand is the perfect damage rune for Tryndamere synergizes insanely well with your[R]. As you pop [R] at low health you'll have a few seconds of being able to deal massive amounts of damage.
Sudden Impact synergizes well with Tryndamere's [Q] when looking to spin in for the all-in kill.
Ghost Poro provides fantastic on-demand lane safety when you're able to keep the enemy shoved in and keep the poro in the river brush uncontested.