(image from WebMD, accessed here: http://www.webmd.com/oral-health/picture-of-the-tongue)
In many yoga classes during centering and sometimes even while doing yoga postures, or asanas, you might be instructed to let the tongue float, release the tip of the tongue away from the roof of the mouth or from pressing into the back of the teeth, and soften and relax both the tip and root of the tongue. Once you do that, the upper and lower teeth stop clenching, the jaw relaxes, and you discover tremendous space in the mouth cavity. On a more subtle level, it's like you shed a heavy coat of tension. You diffuse strain. If you pay attention, you might feel more space overall--from the top and back parts of the head (i.e. the brain, or thinking flesh) all the way down to the pelvic floor, the groins, the hips, and the legs. Your external and internal gaze soften, your receptivity expands, and you realize that the tension in your tongue and jaw has been keeping you on "back-burner high alert," literally keeping you "in your head" at the expense of the rest of your physical flesh.
Modern anatomy provides a simple explanation of that phenomenon. The tongue is the top part of the Deep Front Line (DFL), as explained to us by Tom Myers. The deep front line starts (or ends, depending on how you want to look at it) deep in the sole of the foot (where the tentacle attachments of the tibialis posterior muscle connect and support the major bones of the foot and ankle like a hammock) and ends (or starts) with the tongue. This is literally your plumb line, gravity line, support line, your posture line. Per Myers, it plays a major role in lifting the inner arch, stabilizing each segment of the legs, supporting the lumbar spine from the front (where we all need support!), stabilizing the chest while allowing the expansion and relaxation of breathing, and balancing the delicate neck and the heavy head on top of it. Here is a fascinating video of Myers (note: this a dissection of the deep front line and therefore, viewer discretion is advised), walking us through the entire deep front line in one piece: from feet to diaphragm to tongue!
Doug Keller calls the DFL the core sutra (thread) of the body. And the DFL is indeed your core! Further, per Myers, the DFL is not strictly associated with any movement (minus a couple of exceptions like the diaphragm!), yet no movement is outside its sphere of subtle and profound influence.
But let me get back to the tongue. So, having a tense tongue can literally create tension (and therefore, inefficiency) anywhere along this core line. If the gravity line is not in optimality/balance, the body will find other ways to function by compensating with other more superficial structures (this is the beauty and the curse of the human flesh). Remember this the next time you feel gripping anywhere in the body when in a yoga pose you find challenging or try to do a pose by clenching your teeth and pushing your tongue against the insides of the mouth.
Here is even a more radical thought: why not remember that throughout the day, not just in your daily or weekly or whatever frequency works for you yoga class. Start noticing how much tension you hold with your tongue throughout your day and release it anytime you notice. This can at first be anywhere from exasperating to annoying as you realize how much you do it, but I think the more you engage in that simple technique of letting the tongue free, the less you will grip it. As you do this, there will be more spring in your step (literally!), you will move and function with more grace and utility, and your engagement with your surroundings will be both more enhanced and less harsh.