Challenge Magazine published an article written by Msgr. Vincent Foy in 1988 explaining the Winnipeg Statement and its devastating effects on the Catholic Church in Canada. This is the best explanation on the statement I have come across. It has been reprinted on LifeSite.net
For those of you who do not know, this statement was a response to Pope Paul VI's encyclical Humanae Vitae, essentially rejecting the Encyclical by rejecting the Church's teaching on artificial contraception. To this day the Winnipeg statement has not been retracted.