There seems to be a slight contradiction.
On the one hand, Harry and Hermione find out that one needs to know what a spell does in order to cast ist successfully (Ch 22, “If you didn’t tell her at all what the spell was supposed to do, it would stop working.”).
And then in 26, about the 6th years Griffindor (canon!Harry) who hexes canon!Draco: “He is in his sixth year at Hogwarts and he cast a high-level Dark curse without knowing what it did.”
Shouln’t that be impossible? Or if the knowledge from Harry and Hermione’s experiment wasn’t very general, Harry should have noted at least. (Though honestly I didn’t notice during my first read, being amused about the reference to canon).
There seem to be two forms of leglimency, one that requires an explicit spell and a wand, and can be performed by most wizards. That’s what Mr. Best in MoR uses, and what canon!Snape uses while trying to each Harry Occlumency. The victim knows what’s going on, but usually can’t do anything against it.
The second one is the form that Dumbledore (and canon!Voldemort), which just requires looking into the eyes of the victim, and lots of training. This is the “stealth mode”, and most victims don’t notice the intrusion at all.
It was always my intuitive understanding that the first form allows you to dig deep into one’s memory, wheres the second form only shows you what the victim is thinking right now.
Does that make any sense?