Traditional tree climbing 'frog-style' SRT ascent systems using the hand, chest and foot ascender always left something to be desired in terms of safety.
Toothed cams biting into your climbing line above the primary connection point on the bridge does not seem like a very bright idea once you have seen how easy it is for ropes to 'sock-off' when the sheath is torn.
Diehard cam'ed device SRT ascent proponents will say the answer is simple, never allow slack in your system, and make sure the hand and chest ascender are not 'impact' loaded at any point in time… and you'll be as safe as houses.
But you have to be pragmatic - tree climbers do step up on branches, slack is introduced, and a slip at the wrong time can occur. And that's not including deliberate SRT 'work positioning' on chest and hand ascenders undertaken by tree climbers who should know better.
This is the beauty of the HAAS - the Haul Ass Ascent System by Michael Frankhauser. In Michael's set-up both 'toothed' devices are located below your primary connection point (the bridge of your harness).
Toothed cams below your connection point
In this scenario, should the integrity of the climbing line be compromised by either of the toothed cams, the tear in the rope would be below your attachment point and friction hitch - looking up at the good climbing line above you is decidedly more comfortable than hanging on a torn thread!
One can imagine a time in the future when the lower positioning (below the primary attachment point) of toothed cam devices might be mandatory.
Utilising the full power of the muscles in both legs the HAAS can be employed in DbRT and SRT applications.
Many tree climbers already own a foot ascender so the only additional equipment you need is the HAAS itself (don't bother trying to make one yourself - the splicing and internal components are far more complicated than they look in the video).
There is a little bit of shagging about to get the fitting of the HAAS correct but once that is done you'll wonder how you ever climbed without it!