With Heimerdinger, you can do E->flash normally to surprise the enemy, but if Flash was used late enough, the visuals won't match the hitbox until the grenade hits the ground. This works with both normal E or empowered E, but it is easier to do the empowered one.
1. Use E
2. Wait until the casting bar reaches the last 10%
3. use flash
It might be a bit hard to get it right, but when you get used to it, you can do it 100% of the time.
This looks way worse when thrown upwards/downwards or from fog of war.