When a witcher casts the Quen sign, he casts a protective field around himself that will take damage in the place of the witcher himself. This sign is highly defensive and can be used to help counter, as well as a better form of defence than guarding with a sword.

If Quen is active, the shield will fully protect Gerald from at least one strike. So, the shield can absorb the full strength of a strike that is more powerful than the shield. Because of this trait, casting Quen is invaluable when facing high damage wielding opponents. Note however, that later in the game (particularly in the expansions) enemies have the ability to strike multiple times before you can re-cast Quen.