The universal problem with the general umbrella phrase "support" is that it has many more definitions than most singers and teachers are willing to acknowledge. Support is both the air pressure beneath the cords (subglottic pressure) and the amount of air being released through the cords (glottal airflow.) The vocal cords themselves are also responsible for increasing sublottic pressure and glottal airflow. The twanger also assists in this process.
So, already, upon deeper inspection into "support" you can see that it's more more intricate and systemic than the instruction to "hold your belly out as you sing." To create further complexity the amount of air needed for every phrase, every note, every style, every sound, every volume, etc. can be much different!Some sounds require you HOLD BACK a lot of air and release a lot of air at the same time (loud belting.)
Some sounds require you HOLD BACK a lot of air and release a lot of air at the same time (loud belting.) Some sounds require you to hold back a lot of air but release very little (quiet 'mixed voice' singing). There are many different ways to support and everyone will experience supporting the same type of sound differently!
There are just too many variables to singing as a whole. Support is ONE of those variables; but it's really really over-emphasized. It's a great piece of the puzzle and should be talked about... but so many singers and teachers talk about it as if it's THE method to singing. Sure.. some singers might experience support as being paramount to their technique; but I don't think they are in the majority.
Not only is support just ONE variable in the greater picture of singing (registration, twang, vowels, cord thickness, open/close quotients, etc.), but muscular tension in the "support muscles" is only ONE variable of support itself! Treating support like the powerhouse of the voice instead of a small cog in a larger system is a HUGE mistake.