Hey, bare with me here, this is a bit hard to explain!
When you fire a Fireball and a Pyroblast at the same time, it technically doubles your chance to get further Pyroblasts/Heating Ups. The Heating Up effect doesn't fade immediately after not critting so lets look at it in this scenario:
- You have a Pyroblast and a Heating Up proc present
- You then cast Fireball, and immediately use the Pyro proc
- The Fireball which hits first does not crit
- The Pyroblast which hits second (a few milliseconds after) does crit
What most people would assume is that, you would just end up now with a single Heating Up, because the fireball didn't crit right? In fact because the Heating Up doesn't fade immediately, it allows for the second attack to actually proc the Heating Up into a Pyroblast.
So in 3/4 scenarios you win:
1. Fireball and Pyroblast both crit = You end up with another Pyroblast proc and Heating Up, so you can repeat the process.
2. Fireball doesn't crit, but Pyro blast does = You end up with another Pyroblast proc, fish for a Heating Up proc and repeat the process.
3. Fireball crits, but Pyro doesn't = You end up with another Pyroblast proc, fish for a Heating Up proc and repeat the process.
4. Neither crit = You end up with no procs.
With good luck, you can maintain a rotation of Fireball -> Pyro -> Fireball -> Pyro for a decent amount of time. Another reason to do this playstyle is to use Inferno Blast less, remember that Inferno Blast is your lowest hitting spell which causes your Ignites to be lower. Doing this method will give higher average Ignites due to chaining spells within close timeframes.
Hope this helps!