the vertical plane - the precise plane the spine and the rest of the body is best able to absorb the residual energy.
I am not a medical professional, and will certainly defer to such a person on this, however, until then, I very much disagree with this idea. I accept that there are crush panels in the Cirrus, which will absorb a vertical load, but even so, I would far rather ondergo a 60 MPH deceleration across the ground over many feet, perhaps hundreds if I do it right, than a vertical deceleration at 20 MPH in no more than about 4? feet! (unless a tree cooperates).
A five, four, or at worst three point harness will distribute the decelertion forces widely across the torso if it is a forward or nearly so force. Add to that, an injury resulting from deceleration in that direction tends to be less severe, relative to the "G" force. A vertical deceleration up the spine concentrates all the force up the spine only, and no muscles or tendons can play a part in absorbing that force, as they would were it to be forward into a harness. It's entirely weight on the spine. And, if you do an injury up the spine, it will be much more serious. I expect that experts on ejection from aircraft would have a lot to contribute on this.
Until then, when I'm flying a Cirrus, deployment of the 'chute will firstly be in accordance with the flight manual only, and thereafter, only if I deem the situation life threatening. If an engine failure in this type of aircraft is life threatening, I must have already pushed the limits really far somehow!
I have force landed four times, and (very lucky) not only have I not damaged an aircraft doing it, each time, I was able to fly it out very safely, once the problem was resolved. Though at times I have been required to wear a 'chute, I have never wanted to have one, or considered using it!