Barrel moves directly back with the slide and begins to cam downwards after this point:
But I may be wrong on that one...
Based on my limited time with the pistol, I think the medium back strap mimics the 75B grip angle the best, though none are perfect. The back straps fit quite well by the way, no movement or play on my sample of one.
If at half-cock, DA starts here (after slack is removed):
Although the aftermarket support continues to grow, sometimes you have to roll your own...