@rickyw, let's continue this.
Increasing intra abdominal pressure with a rounded spine is how people herniate discs all the time-bearing down on the toilet, sneezing, etc. In fact Dejerines triad is an orthopedic test for disc-do you have pain when you cough sneeze or bear down?
The devil is in the details. Learning how to spread the load is a skill, and without that skill, simply increasing IAP can certainly do damage. We have no disagreement there. But the same comparison could be applied to many lifts, e.g., trying to pick up something heavy without the total body tension we teach at StrongFirst could, indeed, be bad for someone's back. But the way I practice it, it helps, not hurts, my back. Feed-forward tension is one of the key things about our methodology. It is part of the "reverse engineering what great lifters do naturally" that we pride ourselves on at StrongFirst.
I could go on at length about this but I will stop here - it's another discussion by itself. Suffice it to say that knowing how to get tight and stay tight is key to strength _and_ key to the healthy performance of any challenging strength activity, be it a barbell deadlift or a bodyweight pistol.
But is [the lumbar spine] supposed to be rounded under load? My understanding is that it is not the healthiest thing to do on a repeated basis.
No, it's not supposed to be rounded under load, but we learn from Dr. McGill that at least some people tolerate this pretty well, and a bigger "don't do this!" is _moving_ it into a more rounded position while _moving_ under load, e.g., rounding at the bottom of a barbell back squat.
You actually raise two points - above, we're talking about rounding under load. Your second point is about doing this repeatedly, and that's a separate thing because anything we do repeatedly (by which I assume you mean for multiple reps in one set) necessitates "turning down the volume control," by which I mean lowering the overall level of tension. Because tension is key to the safe practice of strength, we have to be more careful. There are some people who can tolerate doing a lot of pistols in a short time, but there are definitely more people who can learn to do a pistol as a strength move and safety practice it for low reps. I like to think that most people fall into this second category - they learn how to move well (they clear their FMS), they learn how to be strong, and they learn how to spread the load of a high IAP across not only their entire lumbar region but really their entire body, and they therefore can learn to perform a pistol safely.
Despite all this I recognize there are people out there who are getting away with heavy loaded pistols.
As mentioned elsewhere, a loaded pistol allows one to round the spine less, and the load is quite light on the spine compared to a barbell back squat. I think learning a bw-only pistol is more challenging to more spines than learning to do a heavily weight one.
-S-