Good hand technique is especially important when playing acoustically. When you played amplified, because the microphone is held inside of the cup, the effects are much less dramatic. Nonetheless, there is still some effect when playing with a mic. Amplified or acoustic, you want to have a good seal to begin with.
For further study, among the best players to look at are Big Walter and Sonny Terry. Big Walter for his ability to go from beautiful muffled notes emanating from deep inside .. to an incredibly live and vibrant sound when he opened his hands up. Sonny Terry hand effects being highly expressive .. as well as a visual trip.Topics and/or subjects covered in this lesson:
A Harp in the Key of E.
Loop 0:36 Breakdown of Hand Effects
Loop 3:58 Dust My Broom Example for Practice