I understand it this way.....
A crowned sapling bow with elm likes a flat belly, because elm is always stronger in tension as it is so tough. A narrow and thick elm bow takes set because the tough back squashes the belly, so leaving it wide and flat actually balances the tension and compression a bit. It gives you more wood on the compression side to work with, to prevent set, etc... Taking a 2.5" wide bow from a 4" dia sapling works fine. You totally knew knew this.
PearlDrum's osage bow came from a VERY tiny stave, so crown is really high, i.e. tension forces concentrated on a tiny band running down the crown on the back. NOW, osage is only about AS STRONG as elm in tension, but much stronger AND more elastic in compression. So, the belly could resist the back TOO MUCH if made too wide, right? By also crowning the belly, or rounding the cross section, he ensures that the narrow, strained band of tension on the back is exactly opposite a narrow, strained band of compressed wood on the belly. That way, instead of resisting the back enough to endanger it, the belly HAS to compress.
Now, if you did this with elm, the belly would compress too much, since the back is RELATIVELY so strong, and it would take nasty set. But, osage is both stiff and elastic, so even though this might take slightly MORE set than some osage bows, it'll be pretty minimal, and certainly worth the trade-off of protecting the back. And, since the skinny osage pole was pretty long, and he had all but the very middle working, you almost aren't gonna notice.
This is why osage is a 10/10 bow wood, and elm is a 7.5. Not because an elm bow can't shoot awesome, but because with osage you can get away with much more, you have more options, etc...
This works best with elastic woods, vis the yew ELB. Compression strong woods with LESS elasticity, like black locust work great with a crowned stave, a flat belly, and just a slightly thinner and narrower limb than the elm.
