That's cheating
Even if they were sometimes not seriously sparring, Jason was, in most cases, going all in, using his utmost efforts to win.
Without using everything one had up to their sleeves, they would never know how strong they were, and if it might be a stupid idea to fight specific opponents.
Furthermore, Jason hated losing, and even more so after not giving everything he could.
With that thought in mind, Jason's will to continue fighting, and to defeat Jennifer increased substantially, intensifying the ferocious, wild aura he released.
Merged with icy coldness, both his aura and the pressure he was releasing could be controlled freely, which allowed Jason to realise something he had never noticed before.
Normally, the Celestia aura was neutral, and without an affinity, but now while being fused this had changed. Now, he could use the Frozen Flame and his ice affinity, without an insignificant mana consumption.