If Sylas drops under 40% max hp threshold, he gets the ability to case empowered W without any mana.
1. Get Sylas to under 40% health while you have no mana
2. The indicator for W will show animation for the empowered cast
3. Cast the spell onto champion even though you have no mana for the cost of the spell
I've tried this in multiple games, able to reproduce it always. Most of the attempts were with Sylas level 5, 3 points in W spell, 1 in Q, 1 in E.
**Edit:** This is also the case for Nunu & Willump empowered Q
Basically for any spell that has 2-part scaling based on the missing HP
Sylas +/- 40% HP
Nunu & WIllump +/- 50% HP