The description of Genji's Swift Strike states:
Genji darts forward, slashing with his wakizashi and passing through foes in his path. If Genji eliminates a target, he can instantly use this ability again.
However I (and others) have encountered situations where it seems that I am able to kill a Genji mid dash before he reaches me, yet am still hit by swift strike (receive damage).
How does the damage for Swift Strike work?
You would think that damage would be applied as the dashing Genji's player model collides with an enemy players hitbox. However it seems like the damage may be applied to everything in the dash path simultaneously (kind of like a hitscan line attack), with the dashing Genji being purely visual.
UPDATE: Apparently some of the questionable deaths are related to a bug:
Tim Ford (Lead Engineer of Overwatch):...That's a bug with Genji's Dash itself. For a very brief period of time, it can still deal damage after Genji has died. We'll get a fix for that one, soon.