That the muscles under that chin should be soft when singing? Its really tough..lol. And my voice sounds weak when I do it..maybe I need to get used to it.

Haha, but I doubt all the rock and metal singers run 'round singing with soft muscles under their chin.

thats an sls thing. I do not agree. jaime advocates strengthening those muscles and i couldn't agree more

The muscles under my chin are often almost rock hard when i'm in head voice. They tire quickly, but Jim says this is good, so I figure I'm just building endurance.

Hmmm..very interesting...thanks

To make a noise (talking just at the larynx now), you don't actually need any mucles other than the muscles within the larynx (so nothing under the chin) however there are lots of other muscles that help that are connected to the larynx. Jaime talks about the bullfrog exercise in RYV and these help to get a balance between the muscles below and above the larynx as this brings stability to position of the larynx.
If them muscles are 'rock hard' under the chin then I put it to you that something is going wrong. It is associated with 'reaching' for notes and I would suspect that it would lead to you being constricted. You are perfectly capable of hitting the highs without these muscles being rock hard so why waste all that energy doing so? It's energy that could be going into an awesome performance! Just remember that strengthening these muscles doesn't mean you need to use them whilst singing. Tone them up for a balanced voice then leave them alone.
Why do I say this? Because 1 you don't need these muscles to produce an actual sound, so why use them?! And 2, I used to do exactly the same thing and once I trained out the rock hard muscles I became a much better vocalist for it.

I agree with open. Yes Jim says they will get tired and yes I have you doing exercises for strengthening , BUT neither of us ever said those muscles would be rock hard. I hit high notes all the time and the muscles under the chin are relaxed. It's not an SLS thing, it's just correct technique. If they are rock hard, you over over engaging the muscles.

So how do you keep those muscles soft? It seems they harden up on me every time I start ascending into head voice, or occasionally, very rarely actually, when singing quietly in chest voice.

It's counter-intuitive, but strengthening the muscles will actually help them to be more soft and flexible when in repose. VSR and the RYV non-vocal exercises (bull-frog, tongue push-ups, vocal fitness) will also help eliminate some of that tension.

wow...this forum is great..thanks everyone...it all makes sense now.

Singing should be natural and feel relaxing. f you feel any tension, you`re doing it too much.

Yep I agree with Oiselle....that principle is really easy for me to forget...I just wanna get the sound out so bad...but really the trick is to "sing SMART not hard"