Excerpt is from http://nhlcbanews.com/dalymail/daly_mailbag100504.html, the official website of the NHL lockout...
"Ron from Greely, Colorado, asked . . .
Q: Can you explain what 'impasse' means and when they can implement it?
Bill Daly: 'Impasse,' as a technical, legal term, is defined as a point in time in the bargaining process at which the parties are deadlocked with respect to a particular issue or set of issues. It is, by definition, a temporary state during any negotiation because it can be broken at any time by a change in position by either or both parties. A point of 'impasse' in negotiations, by itself, has no real practical or legal significance unless and until management wants to implement unilaterally the terms of its last proposal as new terms and conditions of employment. Unilateral implementation of new terms and conditions of employment is certainly an option for management in any collective bargaining negotiation at an appropriate point in time."
What is to stop the airlines from doing this in the United States?