This was my original thought as well. The prusik could keep the Safeguard from camming while the Safeguard simultaneously tends the prusik. The result would be that in the end neither would hold resulting in a quick trip to the ground. In my rudimentary static tests on my pictured setup I could get the Safeguard to tend the prusik but I could never get the prusik to keep the Safeguard from camming. Not to say it's not possible . . . I just couldn't make it happen.
I do think it’s worth pointing out though, the amount of force generated in a fall on static line far exceeds that of us quickly sitting into it on a tree or in the basement. I can untie a double knot even after sitting onto a rope swing, but if I jump onto the rope swing pretty hard, I can’t untie the knot without a screwdriver or cutting it. So after even a short fall, everything would act different, the rope would stretch (thus temporarily becoming a slightly thinner diameter) because more than 300 lbs of load would quickly be applied, the cam return rate, would be the same speed though because the mechanical rotation would be the same distance. In my experience with both a gri gri plus and the Madrock safeguard they close with slight tension on the brake line side. This includes a short drop I had two seasons ago on an oak I was climbing. I threw it around the tree and there was a small stub branch on the back side. My girth hitch appeared to be tight from below at ground level, but about half way up the tree the small stub broke and I dropped the distance it took for my hitch to cinch the remaining distance. I was manning my brake side over a redirect pulley. I’m not sure of the distance but the rope stretched and absorbed the force. It definitely fed back into the device an inch or two though. Perhaps that’s because of the redirect? Maybe just because that’s what they are designed to do as belay devices? It didn’t hurt at all but it was a butt puckering experience for sure. For reference I was using 10mm HTP and it did well absorbing the short fall force.
The point I was trying to make though is, even in a rappel or climb situation, yes the cam should close that way because it’s a belay device, which is a catch design, but there’s no guarantee it returns fully or catches without tension from the brake line. That’s why they say the brake line must be controlled and it’s also why they have that very first line on the warning stating it’s not auto locking. I appreciate everything everyone added. I do apologize for having the spring function incorrect and for the amount of disagreement it caused. My concern is people reading all these theories and “tests” then going out thinking that either device will cam closed if the brake line is released just because they also released the handle. The manufacturer says differently. I don’t want someone to end up hurt. There is a lot of brand loyalty on this site. The safeguard is a great tool, as is the gri gri plus but neither are truly made for our use which will at some point require hands free use.
Did a couple of unscientific tests with my Safeguard using 8mm Restech (FYI I have never used this combo on an actual climb/ hunt, I always use 9mm).
The first involved putting extreme slack everywhere I could and lightly simulating a fall (while standing and just dropping my body weigh suddenly). By extreme slack I mean 3 things. 1) I hung the brake end over a simulated tree limb so that the tag end of the rope was lined up and ready to slip straight through the device with no friction (no bend) on it whatsoever. 2) I pulled about a 10 inch bite of rope out the bottom of the device (remember I'm standing so there is no weight on the biner). 3) I massaged the device such that the cam was not pinching the rope at the start of the simulated fall (though I left the lever untouched in the closed position). I could freely move rope through the device by pulling on either end with no felt friction. When I dropped my weight quickly and the slack pulled out of the system the cam engaged immediately and caught my "fall". I repeated this several times with varying degrees of supported drop and the device cammed, pinched, and caught me every time.
Test 2 was to do everything above the same, but this time hold the lever in the open position until slack was eliminated during the "fall" and then letting go of the handle. Same results. It pinched the rope and caught me every time.
None of these were full drop shock tests. I am curious if anyone has the setup to conveniently test such. I'd contribute some money to buy a few devices to see how they'd handle a shock load.
As stated previously, I am not an expert or an engineer, and you should not take my word for anything. I am not suggesting that warnings from the company or others here are not valid.
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As a belay device, they are tested to meet a certain standard. I believe it includes a drop test of some form because of its intended use