Knockback
Knockback attacks are attacks that push players back a varying distance. These attacks often deal little-to-no damage themselves, but attempt to push players into potentially-lethal traps and AoEs.
These attacks can often be completely resisted with 
Arm's Length or 
Surecast, but not always. In newer duties (since patch 6.4), knockback telegraphs that cannot be resisted will usually be purple.
Knockback attacks typically require players to run towards the source of the knockback, or position themselves in such a way that they are pushed into a safe zone afterwards.
Warriors in particular should be aware of a duty's knockback mechanics, as 
Inner Release's knockback immunity may interfere with being pushed into a safe zone.
Paladins should be wary of using 
Cover during these attacks, as it will cause them to take two instances of knockback instead of one, often leading to guaranteed death.
Types
Radial Knockback
A knockback originating from either the center of the room or the boss's current position, pushing all players away from the origin of attack. Often indicated by arrows on the ground pointing outwards in all directions.
Players generally want to stand as close to the center as possible, as this attack is typically combined with a damaging edge or hole.
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A radial knockback emanating from the center of the arena. -
A radial knockback that cannot be resisted with abilities.
Directional Knockback
A knockback that pushes players in a single indicated direction, often shown by arrows pointing in the direction players will be pushed. Players should move towards the source of the knockback and ensure they will not be pushed into any dangerous AoEs behind them.
Epicentral Knockback
A blue circle will appear on the arena that will do a strong Radial Knockback, but standing inside the blue circle itself will deal high, if not fatal, damage. Players must stand as close to the circle as possible without stepping inside— also ensuring they're not being pushed into danger.
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The epicenter of a knockback. -
An Epicentral Knockback that cannot be resisted with abilities.
Line Knockback
Similar to an epicentral knockback, but is bi-directional from a line AoE. The line AoE has a dark red telegraph, and standing inside this dark red area deals high-to-fatal damage.