The Australian style of underwater running is unique in that it works irrespective of the runner’s buoyancy and depth of water. This article surveys several different ways of running underwater.
There is a beauty to this slow motion running in a pool! This swimmer achieves negative buoyancy by exhaling prior to submerging.
https://www.youtube.com/watch?v=U0O_vb-ruDs?rel=0
By comparison, the Bajau free diver’s walk is just mesmerizing. The diver achieves negative buoyancy through low BMI, and diving deep enough that the lung volume is reduced by a significant fraction. (starts at the 2:17 mark)
And of course, negative buoyancy may be achieved by carrying a rock!
https://www.youtube.com/watch?v=pOfjHNCkm4I?rel=0
In contrast to these approaches, the Australian style of underwater running does not depend on achieving negative buoyancy! Runners of all BMI’s are welcome, you can keep your lungs full of air, and … no rocks required!