There's a way you walk in Bagua - kind of a glide, pushing off the back foot, making contact with the ball of the front foot, rolling on to the whole foot, shifting the weight forward, and pulling the back foot in, that's called 'mud walking', and though not a necessity - you can quite legitimately walk Bagua heel toe - it is found as a standard practice in many schools.
The soles of the feet stay parallel to, and only just above the ground, and to open the lower back up, you can practice trying to pick the ball of the back foot up before the heel as it steps in. The other key element is noticing the moment the feet are right next to each other, kind of a balanced 'neutral point' where changes of direction can occur.
Walking this way certainly does open up your back, gives you good balance, connects the lower and upper halves of the body, and generally keeps your center of gravity low.
The name 'mud walking' implies that it's for walking through mud ... OK, maybe, but a more inclusive theory is that it is for walking across any uneven terrain where you have to be certain of your footing but cannot necessarily keep looking down - battlefields and blood were mentioned, and this make better sense.
What I do know for sure is that it is a very natural way to move when you are continuously cutting with a large blade whilst trying to move fast - I was practicing moving with a Katana the other day and cutting with each step, and ended up mud walking without even thinking about it ....
I also know it bears a resemblance to the step you use when you are trying to creep up on something, and it's certainly what happens in the dark of night trying to detect and avoid sleeping black dogs and cats camouflaged on dark carpet.