Arm's Length
Arm's Length
“Creates a barrier nullifying most knockback and draw-in effects.
Duration: 6s
Additional Effect: Slow +20% when barrier is struck
Duration: 15s— In-game description
Arm's Length is an action unlocked at level 32. It is available for Melee DPS, Physical Ranged DPS and Tank.
Additional Information
The 
Slow is a reduction to enemies' auto-attack speed and global cooldown time. This makes Arm's Length in effect a defensive ability in addition to nullifying knockbacks. Therefore, it should be used as part of a tank's regular defensive cooldown rotation when pulling enemies in dungeons. Note that the Slow debuff should not be confused with 
Heavy which is a reduction in movement speed.
Not all knockback and draw-in effects can be prevented. In some newer duties, these will be indicated with purple knockback telegraphs.
The analogous anti-knockback ability for 
Magical Ranged DPS and 
Healers is Surecast.