The Phalanx's primary ability is a shield you can toggle on and off--there's no cooldown on this thing. When you pull the shield out, it allows you to absorb attacks without getting hurt, so long as they're coming from in front of you.
The Phalanx's second ability by default is the Shock Pulse. It stuns enemies in a radius around you, making it great for those moments when you're unexpectedly surrounded and your shield isn't helping.
The Phalanx's passive ability, Bulwark, increases the damage your weapons do for every attack you block with your shield, so there's a big incentive to try to keep it deployed as much as possible.