What was the secret to Roshi's power up?

[Warmmedown]

He went from weaker than Tao to stronger than Krillin. How did he do it? Why didn't he do it before, to defeat Nappa and Vegeta?

 

[SSJ2]

Maybe we're supposed to believe he did some secret training for the past however many years. In the FnF manga, he's brought to fight Freeza's soldiers when Yamcha and Chaozu were left behind. Then by the ToP he had pseudo Ultra Instinct and clashed with Jiren lol. Pure comedy really.