You jump whilst looking at the ground and whilst 'ascending' in the jump.. You release the 'shoot fireball' button.
The explosive force of the fireball + the upwards jump momentum is combined and you go further.. That's the 'theory' anyway.
It's kinda channeling an oldschool thing from the 90's / boomer shooters / 'quake' / unreal tournament ... it's basically 'rocket jumping'..
It's much easier to get a 'feel' for it, if your performance is good lol. (and even then, it can bug out on occasion in some areas.. So it may not be you )
Thanks for your kind words though :)