Primal shields are offhand spells used to block projectiles. However, it fractures as it blocks attacks, and will be put on cool down if not deactivated to regenerate. Spells that hit the shield's center deal less damage to it.
Recharging your shield effectively is critical for surviving long enough to start collecting embers. Whenever possible, rest your shield in between individual shots or by occasionally allowing small shots through. If you can't do that, rest it by teleporting out of the line of fire.
You can also use your hand shield against some ember attacks, including Electromancer's overhead lightning balls and Drifter's mines. That means it's important to keep it healthy so it's ready when you need it. If you're in a class that reflects damage back to an opponent, and their shield reflects it again back to you, it can add to incoming attacks to stack up damage to your shield faster. In that case, it may sometimes be better to reflect the shots elsewhere or only attack while your own shield is healthy enough to absorb the bounce-back energy.
Shield management takes practice and an awareness of timing. It's a fundamental skill for game success.
Anarchist: Arcane Shield
A Fiery white circle with a red letter A anarchy symbol in the center. Primal attacks that hit the center bounce back to the caster. With a quick and steady hand, one can hit an opponent with their own spell.
Blackjack: Mystic Shield
A pink diamond with an ornate design. Primal attacks that hit the center bounce back and automatically home in on the opponent. However, it disappears after one hit, forcing you to re-summon it.
Kineticist: Kinetic Shield
An indigo circle covered in purple runes. This shield used to only deflects projectiles straight up, making it useless for counterattacks. However, it is still the sturdiest shield in the game. It's been recently patched in Arcane Update 6.0: 0704 to allow the Kineticist to reflect all enemy primal attacks other than other Kineticist primal attacks.
Drifter: Evil Eye
A yellow circle with an eye motif. While deflecting primal attacks like the Anarchist, (albeit with slightly less health) this shield can be used as a cross-hairs for your spell, and can be used to see embers before they spawn.
Electromancer: Surge Protector
A cyan circle with integrated circuitry. Primal attacks that hit the shield store up to three charges. You can push the shield forward to release the charges as a damaging projectile. Remember that your shield only recharges while it's down, so keeping it up to keep charges stored can prematurely exhaust your shield, leaving you vulnerable to primal and ember attacks.