Jinsung was
about to go all out.
When he was hit in the back by Maschenny.
We never got to see the full extent of what he was capable of.
Why do you think he withheld?
- Kallavan used him as a female sanitary napkin (he was bleeding alllll over, coughing blood, and his arm was "a complete mess" - his words - as it dangled there - and not a scratch on Kallavan) prior to
Crouching Tiger, Hidden Dragon - low chance he was withholding... unless getting the shit kicked out of him was all a part of his genius plan!
- Jinsung's inner dialogue pointedly stated he MUST do ANYTHING to stop Kallavan after seen EoB released to keep him from going to Baam, who he had put his life on the line to protect. His exact words: "I knew it! I made the right choice... no matter what happens, I can't let this lunatic get to the last station!" - even lower chance he was withholding
- Jinsung put it all on the line with TigerDragon - which he thought was going to be fatal to Kallavan (it didn't even really slow him) - then, he proceeded to getting his 'lethal' arms taken from himself like Kallavan was picking flowers roadside. Kallavan had MADE SURE to keep Jinsung from doing anything else that could have hurt him by destroying his arms, which were shown to be mincemeat BEFORE he got shot in the back. This is the main reason why he used RFC with his
feet and didn't do any more assaults on Kallavan.
Kallavan blatantly told Jinsung he could have killed him if he had followed up with another hit. Jinsung clearly couldn't (or he would have, specifically being told as much) as he was already gassed and his arms were destroyed, or both most likely. He was left with only RFC from his
feet. If he could have done anything at that point he should have made sure to do so, but his options were limited. As impressive as that was, he was hoping the blast from El Robina would have stopped Kallavan (and probably ended himself as a suicidal hit) as that was his LAST option. Point being if he could have ended Kallavan with any certainty with his own body and stayed living, he would have. But, he couldn't so he didn't.
We have no indication that Jinsung left anything on the table, and every indication of the opposite reality.
IF Jinsung was holding back (which in every way does not seem to be the case) then he's an IDIOT and that would betray his own thoughts and goals to save Baam, and even himself being potentially caught up in an anti-HR El Robina/Capital ship explosion. And, IDT Jinsung is an idiot.