It's a pretty simple idea. Ideal riding position when seated is an aligned pelvis, back and neck, loose, relaxed arms and loose, straight wrists with a full, easy grip on the handlebars. A rider in this position can use their abs and skeleton to support their body weight, rather than their wrists and elbows. Have you ever seen a woman riding who is trying to ride with her fingertips on the grips instead of her full hand? Has anyone ever complained to you in your beginner classes of sore wrists, elbows, neck? In almost every case, this is saddle/pubic bone related - the rider can't align their pelvis with their spine when seated due to saddle discomfort.
Look at the two figures below - the left figure shows what happens when there is an impediment to pelvic rotation - their back is hunched, elbows locked. The right figure shows a rider with a pelvis aligned to spine. They are able to relax their arms, shoulders and neck. They can also look down the trail instead of at their tire. You can actually try this yourself while seated on your bike - rotate your pelvis back as if you don't want your pubic bone to touch the saddle and then rotate forward and align your pelvis and spine. It is really easy to see how a comfortable saddle/chamois gives you up to two inches of extra, easy reach to the handlebars.
There are a few things that can cause riders to rotate their pelvis back, into an unaligned position:
- No chamois
- a hard saddle nose (typically found on non-women's saddles)
- a really really really soft saddle that does not offer pelvic support
- unusually weak abdominals
- UTI or other medical issue
In many cases, when riders get onto a WSD bike for the first time, it is also the first time that they are getting onto a women's saddle. The huge increase in comfort and the aligned body position feels great and they buy the WSD bike.
What new riders don't know that you can improve your fit with a women's saddle on almost any bike, WSD or unisex, and that it greatly increases shopping options to test ride every bike with the same, good seat. A quality shop will do this happily.
The consequences of poor pelvic alignment go a lot further than comfort. It is also a huge factor in seated descending and cornering ability - an unaligned pelvis/back effectively moves your center of gravity back and takes weight off of the front end of the bike. This causes instability and significantly reduces front wheel traction. Many women who think that they are poor descenders/cornerers are actually just not in a good position on the bike - road and seated MTB in particular. You HAVE to put weight on both wheels to corner effectively
(for more on great MTB technique, check this video series from Fabien Barel. Skills + hot French guy bonus! Applied diligently, these techniques will change your riding forever).
All that said, some bikes are just plain better riding than others - particularly in the WSD categories (because many of these bikes are designed more for a beginner's test ride than they are for an experienced rider on the trail). You are a really experienced rider and I doubt that you are riding around all hunched up. It may be that the LIV you are riding has a geometry better suited to your riding style than your previous bike. Or that the suspension is set up correctly and your previous bike wasn't (the number of people who don't know that suspension is something that requires frequent setup and checking is something else that we in the industry need to work on - if you own a full suspension bike, you need to own a shock pump or your bike will never ride the way it was designed to and that is a real shame).