When melee swing doesn't land in that window (i.e. the trait works as it should):
http://i.imgur.com/PJqrH3V.jpg
When melee swing does land in that window (i.e. when the trait is absolutely useless):
http://i.imgur.com/l6RIiC1.jpg
Extremely unlikely? Well, feel free to have at it on a tanking dummy and see for yourself. It happens quite regularly.
It has the potential to be an extremely powerful trait, since yes, it gives a 50% leech from the damage of DS. But it breaks if melee swings land in that window.
Not sure how it means I'd be 'using DS wrong' either. I drop to x%. I hit Death Strike. If a melee swing lands during the (brief) window between Death Strike healing and Death Strike doing damage, the trait provides no leech whatsoever. Anyone at all can replicate this on the beta with a tanking dummy. Go ahead. Hop on the PvP realm, make a Blood DK and go swing on the dummy. It'll take you less than 20 seconds to see it for yourself. It occurs quite frequently. It'll happen even more frequently when you're fighting multiple enemies.