Indirect pressure is applied on a command given (and complied with) AFTER the previous command was given and not complied with.
Example: dog is on a point and handler casts dog "over" into water. Dog instead digs back and stays dry. Rather than handler giving an "over" command again, and immediately giving pressure, (Direct). Handler instead stops dog with a sit whistle. Dog turns, sits, and complies with the sit. Handler then gives a nick (indirect) even though pup sat as commanded. The indirect pressure nick on the sit command is for non-compliance with the previously given cast, which pup refused.
Make sense yet?
Forget the stick in my Amish example. The stick has nothing to do with it. The reason I brought up the Amish correction is that I was using indirect pressure (with crummy timing) without understanding it. Dog gets a cast. Dog refuses cast. Dog is given a "sit" whistle, dog complies with sit whistle, but handler goes out there and corrects, not for the sit, which was complied with, but for the previously given command, which got no effort.
Here is the difference. Indirect pressure involves pressure given after the dog complies with the "sit" command. The dog is given the "sit" command AFTER the dog fails to comply with the command issued previously. The most common example I can think of has to do with cast refusals.
The thing that is tough for some of us (myself included back when I was trying to digest this stuff) is that the dog is complying with the sit whistle, yet he gets corrected. Doesn't that confuse the dog? The answer is that when chained together correctly, the indirect pressure helps clarify to pup that he needs to focus and put forth effort on the command given.
How does it work? My answer is that it works great! Some want to know how the internal combustion engine works in detail. Others just want to know how to operate it and then go make it run.