To correct the other Solution by
NickMon68.
The clash system is not randomized and you don't get lucky picking the "right one".
The clash system is where you wager a portion of your meter, you need to wager more than your opponent to win the clash, I recommend ONLY using your meter if you have more than your opponent because the AI usually likes to use ALL their meter when activating a clash (an excellent opportunity is after the enemy has use their super move).
The AI loves to use clashes when low on health so you'll see these quite often, but if you want to initiate a clash yourself you have to block towards the enemy whilst they're in the middle of a combo on you (simply 2 hits or more in a row).
When the enemy uses a clash, winning it will deal damage to them whilst if you activate a clash yourself then winning it will award you with a heal. The amount of damage/heal is based upon how much more meter you have waged versus your opponent. In the case of a draw nothing happens except for you both being knocked back.
Also note you CANNOT activate a clash until you're on your final health bar, the first health bar will not allow you to use a clash, additionally you can only activate a clash once per match.