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One-handed action refers to actions that can be performed on the move while reloading, charging or firing weapons simultaneously; allowing the Tenno to perform a wide range of actions without being constrained with animation locks. For example, a Nova can sprint and cast Antimatter Drop while reloading a weapon at the same time, while a Vauban can deploy Bastille while charging a bow simultaneously.
Two-handed action refers to actions that lock the Tenno in an animation lock, disabling mobility and other one-handed actions. This means that even though Mag's Pull is a one handed action, it cannot be cast while rolling, as the latter is a two-handed action. Likewise while performing melee combos, an Oberon cannot cast Smite, as melee counts as a two-handed action.
List of One-Handed Actions[]
Most Maneuvers: crouch, sprint, slide, jump, double jump, bullet jump, ledge grab, wall dash, wall latch, vault, and ziplining
Nova can sprint and cast 
Antimatter Drop while reloading a weapon at the same time, while a 
Vauban can deploy 
Bastille while charging a bow simultaneously.
Mag's 
Pull is a one handed action, it cannot be cast while rolling, as the latter is a two-handed action. Likewise while performing melee combos, an 
Oberon cannot cast 
Smite, as melee counts as a two-handed action.
