To tuck or not to tuck?
"Tuck your tail bone," "draw/hug the belly to the spine," "lift up through the navel," are all terms commonly used in Yoga class, attempting to guide students to use Mula Bandha and Uddiyana Bandha, your pelvic floor and lower core muscles. Bandha translates as lock and refers to tightening or holding. The Bandhas are integral to the Yoga practice for postural alignment, strength, discipline and lightness.
There has been some controversy of late around the use of these terms and whether they are in fact safe. If you have a posture where your pelvis is naturally tilted forward, meaning that there's not a lot of bend in your lower (lumbar) spine and sometimes have a posture that rounds forward overall, then tilting further forward might not be the best idea. On the contrary those with "banana backs" who have a pelvis that tends to sit backwards might benefit from a little tale bone tucking to lengthen the lower spine. However even people with this body type should be careful not to over do it.
Discussions among senior Yoga teachers and specialist anatomical health practitioners document sensible reasoning for and against the method. My advice is to identify your posture and if your pelvic bones sit low i.e. pelvis tilts backwards, (booty sticks out, like me) work on lifting your pelvic bones toward the navel and drawing the tailbone downward. If your pelvic bones already sit quite high then concentrate on keeping it in that position as you move through the postures, all the while squeezing the pelvic floor muscles (Mula Bandha) and activating your lower core (Uddiyana Bandha). The pelvis can be likened to a bowl that it is full of liquid that you don't want to spill as you move through the Yoga postures.
Everyone's body is different so start to experiment mindfully, paying attention to how the activation feels, ensuring that it is the muscles you are activating rather than straining the skeleton. Keep asking questions - your Yoga teacher, physiotherapist, osteopath or equivalent can speak to you about your posture. Most importantly, don't take anything too seriously in the Yoga practice as long as you are being safe. Life is serious enough.