Deleted member 7654
Good video, which nicely answers my questionThe recurve riser has a plane defined by the grips pressure point and two limb bolts. You can align the limbs straight from each other (at brace) but have the grip's pressure point a smudge to the left of right.
When you start drawing the risers will straiten out, twisting the limbs slightly. Everything (grip pp, limb bolts, limb tips, string) is on or close to be on plane at full draw.
Imagine the rotational forces at work when you release from the untwisting limbs and the impact on string/nock.
There is a whole discussion going on about this on Archery Talk which also contains this video:
I like the method used by tabashir in post #13 here better than the "measuring off the riser" part in the video.
Of course it assumes the riser is all machined dead square relative to that plane and presumably that is the difference between a top quality riser and a cheapo knock-off one?