That's Genos going beyond even a normal full power incineration cannon by equipping his core.
When Genos uses his core to amplify his attack, he's ends up running out of energy and can't fight any more - at least temporarily.
Like how Genos can charge up and fire his
Spiral Incineration Cannon and then continue fighting. But when he uses his core for the
Ultra Spiral Incineration Cannon, he's
taken out of action.
In his sparring match with Saitama Genos can fire
his incinceration cannons at maximum output without using his core. (
Clarified in the databook here as maximum output. It even shows the image of him using it in his fight with Sonic alongside it, so we know he's using Full Power Incinceration Cannon in both scenes and it wasn't his new parts that enabled him to do that)
So; Genos with Core > Full Power Incineration Cannons > typical Incineration Cannons.