I havent tried it myself, but i am proud to say that i AM the product of gender selection via the timing method. My parents had 2 boys and wanted a girl. Mum followed the theory that female sperms last longer but are slow, whilst boy sperms are fast but die faster. The theory is that if you TTC before as far before ovulation as possible you increase the chances of a girl. It worked to conceive me anways
Also, my sister in law conceived by accident a girl in the same method!