While others struggled to gather enough Internal Energy to fill up their Dantians and slowly crawl toward the next stage, Leo and Yeon always suffered from the exact opposite, they always had too much energy beyond their initial needs!
In their case, having a very solid foundation made breakthrough much harder, so they needed a lot of energy to force their way through to the next stage. As a result, they always absorbed more energy than their Dantian could hold at the point of breakthrough.
In the eyes of bystanders, such an act could be considered overkill. Using time to cross this hurdle made much more sense than being wasteful, but to Leo and other 'geniuses', time was their most precious resource.
'If only I could just attach another battery.'
In order to make sure that nothing went to waste, Leo had always initiated Dantian Expansion right after his breakthrough, and so did Yeon. Still, he couldn't help complaining about his always cramped Dantian.