When you bring an enemy's life low enough to candify them, and the menu pops up, Buu continues to fire the candy beam. While the menu's up and you don't candify them, the enemy remains alive. Since the beam continues to fire, Buu continues to gain PL from just continuously beaming the enemy. You can also recharge your ki during the candying, the menu doesn't disappear, and the beam restarts again after you release the ki recharge button. So basically you can constantly keep the menu open and the person almost dead just by recharging ki when you need to.