How do I drop units from overlords/medivacs while the transport is still moving?

Solution 1:

Click where you want your transport to move too, and then while it's moving click on a unit to unload, this works for me. I'm not sure if there is an automatic way of doing it or not.

Solution 2:

I believe Viper_Sb's answer is correct for EXACTLY what you are asking for, but I believe this is worth mentioning:

You can do something similar by ordering the transport to drop all units (D?) and then holding the shift key to queue up a move order. The transport will move on as soon as it is empty.

EDIT: See Raven Dreamer's comment on Viper_Sb's answer for more details. He is faster at composing than I am!

Solution 3:

Okay! You can easily replicate this behavior either manually or with waypoints.

Manual Method: Order the dropship to move somewhere. When the medivac is above the desired location, click the unit's silhouette to unload them mid-flight. The move order for the medivac won't be cancelled, but the unit will unload.

Waypoint Method: Order the dropship to move where you want to unload the unit in question. Shift-click on the unit's silhouette to queue an unload command. Then shift-click another location to queue a second move order for your medivac. This will result in your medivac moving, dropping the unit(s), and continuing to move all in a single, smooth action (i.e., no loss of speed due to deceleration). You can also queue the "unload all" command this way, though that will cause the dropship to stop momentarily.