A raised shield can be immediately canceled into grab at any time. While holding the Shield button, simply press the Attack button too. Shield grabs are primarily used to punish shielded attacks. For many fighters (generally those with close-range grabs), this is their fastest punish. However, most fast close-range grabs suffer from short reach, and there are no grabs in the game that cover behind the grappler, so spacing and position awareness are vital to using shield-grab punishes well.