This goes for any medigun. A sudden flash with the Kritzkrieg can be enough to save you and your patient from a surprise heavy attack. The Quick-Fix's ÜberCharge can heal you enough to withstand any incoming damage while retreating, and can keep Pyros from air blasting you away from safety. The Vaccinator can also keep you safe from random critical attacks, or crit-boosted enemies.
Bonus Tip: When you stop healing a player with your ÜberCharge activated, they will stay invincible for a short while. This means you can charge two players at once if need be. However, doing this will cause your ÜberCharge meter to drain faster, so don't go trying to make your entire team invincible every time you get the chance.
The optimal time to activate an ÜberCharge is right before coming within range of a sentry nest, or entering an area heavily occupied by enemies. However, sometimes you'll get attacked or put into a dire situation such as a Pyro or Scout swooping in behind you or your pocket gets killed before you get a chance to use the charge. In this case, your best option is to activate the ÜberCharge prematurely to keep you from dying, and letting your pocket take out the threat, push as far as you can with what's left of the ÜberCharge, or allowing yourself to make a break for safety.
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