The sensation of flexible firmness is related to both the registrational adjustment and the breath.
First, the registration – which is the balance between the two main muscle systems so the vocal cords are able to be accurately adjusted. When these muscles are in a healthy balance we get the feel of standing on a step. The feel of the step is not rigid nor is it mushy. The feel is of flexibility. We can move freely as our muscles are toned and agile and responsive. The feel is also of firmness. We are “supported” by our muscles being strong, and so we do not have to worry about “falling.” Thus, no need for excessive volume or breath blasting to “hold us up.”
Second, the breath. We breathe in oxygen, the cords close for a split second, and then we sing with carbon dioxide exiting. When the registration is correct for the pitch, volume, and vowel, the cords let out how much air needs to go out. When our lungs are full of air, and as that air moves out, there is a sense of flexible firmness. All is controlled by healthy registration. We neither feel mushy nor rigid in the area of our epigastrium, abdomen, lower back, etc. We do not have to stuff ourselves with air, nor actively tense our muscles in this region. We can trust that the registration will let the air out according to the principle of automaticity, and that the muscles associated with breathing will respond naturally to this process. No direct voluntary attempts to control the breath are needed.
Feel the feel and notice that when all is good at the level of the cords, there is a flexible firmness in the physical area of breathing. Go ahead, breathe a sigh of relief for not having to try to micromanage muscles and airflow!