Well:
"When the players got stronger they found that playing with "completely free" rules was in great favour of Black (the beginner of the game). It is also proved that there is a sure win for Black when playing with "completely free" rules."
(from http://www.renju.nu/r1rulhis.htm)
The rule "no-more-then-five for black" is one of "black" restrictions in renju.
(Cacher) Si vous attendez que ce soit à vous de jouer, cliquez sur "changer" à côté du "rafraîchissement" sur la page principale, puis réglez le rafraîchissement de la page sur 30 secondes pour que votre tour de jouer apparaisse pplus vite. (Servant) (Montrer toutes les astuces)