So, suppose you were looking at a horse from the rear, across the tail. The back of the saddle is curved like the horse. If you look at the Iron Pony (bike ) the same way, the saddle is basicly the same sort of curve. Walk behind the bike and squat down so it's at eye level, look across the back tire.
Now go to terrybicycles.com and look at the Butterfly, or criuse your local bike shop. Different saddles have different shapes, including the curve of the back of the saddle. Some are deep crescents, some are totally flat across the part where your sit bones go. And many variations in between. The shape can be very subtle.
I have a long history of lower back pain on the bike, which I now know to be sacro iliac trouble. When I started riding again after a three year break I had to take the hard racing saddle off the bike and get something a little more easy going, by luck it's totaly flat as a board across the back, my SI joint hasn't flared yet. A light bulb went off. ( This is a big cushy sprung saddle, the kind we laugh at. Laugh no more, it's comfy!)
Hope this explains it. The nice ladies at Terry tell me the flat across the back saddle is popular with other people with lower back troubles, and my physical therapist thinks it makes perfect sense. Some people just hate the Butterfly/Damselfly flat thing, but we're all built differently.
The saddle thing is just such a *****, so expensive to keep buying them and find out what you need or hate. I can't beleive nobody's ever researched what body types like what saddle types. It could never be totally precise but I'd think they could throw us better clues.

Miss Liz

Trivia- the ladies saddle with the cutout is actually a late nineteenth century invention. The Safety Bicycle, what we basicly all ride variations of today was a major factor in the womens suffrage movement. Zooming around the parks in bloomers was called "scorching", absolutely scandalous behavior and totally the rage.
So we get it from our great great grandmothers! Raise that beer to the old girls after your next bike adventure.