 The NHL tried to block several of the defections . The Bruins attempted to restrain Cheevers and Sanderson from joining the WHA , though a United States federal court refused to prohibit the signings . The Black Hawks were successful in having a restraining order filed against Hull and the Jets pending the outcome of legal action the Black Hawks were taking against the WHA . The WHA was eager for the court action , intending to challenge the legality of the reserve clause , which bound a player to their NHL team until that team released him . In November 1972 , a Philadelphia district court placed an injunction against the NHL , preventing it from enforcing the reserve clause and freeing all players who had restraining orders against them , including Hull , to play with their WHA clubs . The decision effectively ended the NHL 's monopoly on major league professional hockey talent . 
