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.
(kaŝi) Se vi volas trovi kontraŭulon kiu ludas simile lerte kiel vi, rigardu la paĝon 'Taksaro' kaj en ls subpaĝo de la dezirata ludtipo trovu ludanton kun simila BKR. (pauloaguia) (Montri ĉiujn konsilojn)