Shield-shifting is performed by holding the Shield button while gently tilting the control stick in any direction. This nudges the shield’s curve accordingly. Pressing too far results in a dodge or roll, which can be avoided by shield-shifting while holding two Shield buttons (ZL+ZR together). This disabled dodge moves and allows maximum shield-shifting.
Shields only cover all of a fighter when they’re fresh and haven’t been held up too long recently or chipped at. If shield size is depleted at all, your fighter has vulnerable portions sticking out somewhere. Attentive shield-shifting in these scenarios can save you from glancing hits. See defense.