Sometimes teachers tell students to support a note from the diaphragm. This isn't technically correct since, when you breathe out, the diaphragm simply relaxes back to a dome shape. The diaphragm only "works" ("contracts") when you breathe in.
All fine and good but ....
Remember Self 1 and Self 2 from the Inner Game of Tennis? Self 1 thinks, Self 2 does. Try as you might you'll have very poor control of your body if you try to consciously control each muscle.
Instead, your Self 1 thinks of of an external goal which Self 2 interprets into a series of coordinated muscle movements.
Self 2 is a black box, really. We send instructions into the black box and some sequence of muscle actions comes out of it.
What instructions should we send? Well, ideally, it should be the sound we want .... the sound we hear in our heads.
But when you get right down to it, any instruction can work IF if it results in the right outcome.
In theory, you could train yourself to think of a chocolate milkshake and, when sent into the black box, this might result in a warm, smooth legato sound in the low register.
You could think of anything..
ping pong balls,
waves on the ocean,
wind blowing through a spooky cave,
blowing out a candle..
If the thought gets a nice result out of the black box, keep it.
suppose you tell a student to support from the diaphragm and they, in fact, are supporting from the *region* of the diaphragm but actually are using abdominal and intercostal muscles.
What really matters is that the thought of supporting from the diaphragm produces a successful result. No, they aren't actually doing it but, thinking of it this way might cause them to play well.
If so, keep it and move on.
So, while I don't tell my students to support from the diaphragm, I'm not quite ready to blast teachers who say this. Thinking of it this way may, in fact, help.
Of course, it could also hurt.
So be prepared to conjure up some other image in the hopes that the black box will do what we want it to do.