Diaphragm vibrato primarily makes use of the diaphragm for altering the amount of air flowing through the harp, which provides a rhythmic pulsing of volume from softer to louder to softer. Throat vibrato may be the most emotional of the vibratos by making use of pitch changes as well as volume changes.
The key to throat vibrato is a smooth oscillation of both pitch and volume. Done poorly it sounds chunky, and not smooth. Done well it sounds smooth and natural and wonderful. Strive for a sine-wave type continuous smoothness, with consistent pitch and volume changes. Rhythm is an important part of the vibrato. You can do vibrato at different speeds, so you choose a speed that fits with the rhythm and feel of the music. The pulses of the vibrato should be made to divide the notes into even intervals to add to the rhythmic content of the music.
The classic way to get throat vibrato is to imitate a rapid fire machine gun (eh eh eh eh eh), like when you were a kid. Then do it inhaling instead of exhaling. It's the same throat motion that gets the throat vibrato. Work on it until all the chunkiness is gone, and it sounds as natural in your play as it does in a singer's voice.
A good way to get the feel is to put vibrato on the 3 draw 1/2 step (or even less) bend. Your throat is involved getting the bend initially, so there's some feel there before you go for the vibrato. Breathing from the diaphragm will help control the vibrato. There's always an interaction between the throat and diaphragm when doing a vibrato, since each is involved in controlling the air stream. For throat vibrato, obviously the emphasis is on the throat--but you'll probably notice some involvement of the diaphragm as well.
Try it amplified, but play softly. Practice very soft draw bends. At some point you'll notice what feels like a direct connection between your throat and the note. Every little nuance of throat motion is reflected in the sound. Work on playing the 3 draw 1/2 step bend softly. Then put some vibrato on the note by pulsing the air stream with your throat. The pitch will change up and down because you've got a hold of the bend with your throat. Keep at it, it's worth it!
How much should you use throat vibrato? Different players like different things, so it all depends. Often it is good to emulate what singers do, and start a note straight, then add vibrato toward the end of the note. Some players think there's no such thing as too much vibrato, they love it that much. Other players like to use it more sparingly, to add variety and contrast. I think you should use it a lot, but not all the time. Throat vibrato is one of the best things you can do to improve your tone, your control over the notes you play, and your focus on the connection between your playing and the response of the reeds.
