In Fire Emblem Fates, the skill is learned by flying Hoshido units. The Speed of the unit is increased by 5, likely triggering double attack.
Fire Emblem Heroes
In this game, it is a Type A passive skill that grants increased speed if they initiates combat. Staff Users cannot learn the skill. Stage 1 can be used to learn Sturdy Blow, Swift Strike and Swift Sparrow
Darting Blow 1 - 50 SP - Spd+2 if unit initiate the attack
Darting Blow 2 - 100 SP - Spd+4 if unit initiate the attack
Darting Blow 3 - 200 SP - Spd+6 if unit initiate the attack