Keyring wrote:Oh I forgot to say. First break off the head of a key while it's in the lock!

bahh... yup... it is important to have a broken key...
Generally, correct... The idea is that the extractor can be wedged into whatever is jammed into the lock and used to pull it out. Depending on the situation, you may also need to lift the pins with a second pick that may interfere with the extractor or the object you are extracting.
FWIW: I found that bending the 'hook' of the extractor a bit so that it grabs or bites into the object stuck in the keyway helpful. It is simply a fishing expedition. Hook it and pull it out.
z