When you hit someone with a skill or basic attack, a ring appears around them. On the 2nd skill or basic attack, that ring will be bigger meaning the third skill to hit the enemy or basic attack will trigger his passive which does bonus damage and gives you the movement speed while slowing the enemy.
So you can combo the passive in different ways..
Try to land Q and make sure it connects on the first and 2nd hit, this allows you to proc the passive from using the E and blinking in to hit them, or proc the passive from basic attacking without using E if they are already in your face.
Sometimes landing the W and stun can be easy sometimes if you were to hit them with the Q both ways, then throw down W behind them, and proc the passive with the E in which slows them as they should run from you and right into your trap.
Anyhow, the 3rd attack does the bonus damage, there is a ring that appears at their feet which tells you if the passive is gonna be procced or not.
The cooldown of the passive should be 3 seconds, so you can start stacking again after 3 seconds I believe.
You can proc the passive in many ways, you can dash in with E first, then Q, and basic attack which procs the passive, and slows them as the 2nd part of your Q comes back and hits them.
The best way to trigger the passive tho is by trying to land both parts of your Q, and then throwing down W, and then blinking in with E. If you know you won't stun them, you can just run back out and repeat.