https://cdn.**********.com/attachments/504814911159795723/844891221611773972/E103WCPUcAAEY4d.jpeg
This is how the Probability Manipulation works. The above is a Charge Impact battle that happens at the start of every fight in DBH, those two bars at the bottom move back and forth. Your goal is to have your bar end up further to the right than your opponent's is left, if you fail at that then the opponent's guards and the damage they take is reduced. The Probability Manipulation comes from either slowing down your Charge Impacts speed at which it moves back or forth or speeding up your opponents, thus affecting the likelihood of you winning and them not being able to guard.
Oh also, if the attacker has a Super move available (Kamehameha, Special Beam Cannon, Final Flash, etc) and they fail the Charge Impact then they won't use that move. Other than that I can fail a Charge Impact as an attacker or win as a defender but the damage is still dealt, it's just reduced.