This is the "professional"
attacking short shot.
Once you are able to hit good lengths from anywhere on the
court and the opponent is not giving you many easy loose balls,
the next best way to pressurise is to play a straight drop
from deep - generally from the technically easier backhand
As with all variations of shot, this will not only create
doubt and thus make your lengths more effective but, even
if it is played fairly obviously, it will usually result in
a weak reply, leaving a good opening for a straight or cross
court drive. (Note: Always keep playing a high percentage
of shots to the back of the court.)
A weak reply to the drop from deep will result because if
the shot is played at the correct narrow angle (Shot A - as
always, miss the nick too narrow rather than too wide) the
opponent not only has to scrape the ball off the wall with
only the very end of the racket, but the ball will also be
in front of him - rather than at his side - making a straight
reply very difficult and a weak cross court more likely.
The same principle also applies to volley drops.