My understanding is that the numbness is due too excessive pressure on the nerve and the blood vessel that run through the soft tissues that rest on saddles.
I recommend taking a look at Selle SMP's patent application, which described their approach, and the ismseat.com website, which described theirs. In brief, SMP has longitudinal cutout that should remove pressure from the nerve and artery, and some curves that allow the critical 'soft tissues' to ride free of the saddle. That worked for me.
ISM's approach is a similar groove and a very much shortened nose, which lets the soft tissues ride free of the saddle. With ISM, however, one tilts one's pelvis forward and rest all one's weight on one's pubic rami. That's too much weight fro some people, which leads to saddles sores. But note that both ISM and SMP have one's external genitalia ride over air, not the saddle. That makes sense to me.