Drop assassinations in Dishonored 2 include both kills and knock-outs. Whenever you get the opportunity, take out an opponent by dropping on them from above (hitting the button for kill/knock out, depending on your play style). You could easily complete the game without ever getting 10 (particularly if you are really focused on stealth and avoiding, rather than eliminating, enemies), but if you focus on getting these when you can, you'll also easily get 10 by about the fourth mission.