Every once in a while, usually in a serious duel, you'll see a witch or wizard perform a spell without saying anything. If I remember correctly, it's usually Dumbledore and Voldemort, but I'm pretty sure I've seen Harry do it, and I think I remember Mrs. Weasley performing a non-verbal spell in Deathly Hallows 2 which totally obliterated Bellatrix Lestrange. How do these non-verbal spells work? Are there certain spells that can and can't be performed non-verbally, or does it depend on the wizard?
According to Harry Potter Wiki,
Performing spells nonverbally is very difficult and requires a good deal of practice, as it requires concentration and mental discipline alone.
That's pretty much all it says though. Is it really as simple as "you have to concentrate really hard", or is there anything more specific you have to do?