good question;
in a general sense, there is a principle known as reciprocal inhibition, based on agonist / antagonist relationships of muscles; in other words, if you fire the biceps by bending the elbow, the triceps, elbow extenders, have to shut off in order to allow the elbow to bend w/out resistance; the harder the biceps contraction (e.g., the heavier the load they have to move), the more “off” the triceps get;
this principle is used in manual therapy all the time, be it in active approaches like Proprioceptive Neuromuscular Facilitation (PNF) or Muscle Energy or passive ones like Strain Counterstrain (go look 'em all up for detailed explanations if you are curious);
in your specific case, whatever it was in spasm probably got turned off by the contraction of the muscles doing the loading - in the second case, dead lifts, that is pretty easy to explain: usually “back pain” has a lot to do with some combo of hamstrings, piriformis, iliopsoas quadratus lumborum and / or lumbar spine paraspinals (erector spinae) being in spasm: a strong recruitment of glutes, quads and transversus abdominus, among other anti-gravity extensor muscles, will create inhibition of the former, w a concomitant decrease in pain, etc.; of course, this is a generalization, I’d have to look at you specifically to figure out what was what - there might be other considerations in terms of how your sacrum, iliac, pubes, coccyx and lumbar spine joints were restricted at the joint level; also, pelvic floor and respiratory diaphragm are often drivers of low back pain; however, the aforementioned heavy recruitment of postural extensors can resolve stuff in all those areas (it’s sort of a “shot gun” technique approach - hits 80-85% of what’s going on)
as regards the leg curls, that’s a little trickier to explain bec usually that is pure hamstrings, and recruiting hamstrings is pretty much the opposite of what one typically wants in cases of radiation; one possibility was that your radiation was due primarily to unilateral piriformis spasm / irritation of sciatic nerve passing through it, and that when you did a unilateral leg curl, it somehow caused release of that via recruitment of some combo of pelvic stabilizers that shut piriformis off; another possibility, since you liked extension but didn’t like flexion, since you were in prone extension doing the exercise, you were positionally taking the back extensors off stretch / shoirtening / defacilitating them, and then on top of that, loading them during the exercise, resulting in a post-contractile inhibition that turned them off even more…I don’t know, it’s hard to say, I’m just throwing out ideas - but the take home message is that it’s not implausible; and also, it underscores how (correct) movement goes a long way to relieve back pain more than just laying around doing nothing does, especially if it’s primarily joint / muscle generated pain (w disc / nerve root, it’s a little trickier, sometimes movement helps, sometimes it doesn’t)
BTW, I am very concertedly working to open up a manual therapy practice down by me starting part-time this summer or at latest early fall (just took my first “refresher” manual therapy course last weekend, yay), and hopefully will go full-time within a year or so, so I’ll be stopping by w business cards at some point (and I am strongly believe in a “gung fu” sliding scale for those in the arts who for a legitimate reason are unable to pay regular rates)
anyway, food for thought…
