During the last TTC strike, I wrote a little backgrounder on back to work legislation and essential service legislation -- how, to my understanding, they actually work, regardless of the hype. Here's the relevant chunks on back to work legislation for those who are concerned McGuinty might set the terrible precedent of interfering in a university labour dispute:
Back to work legislation, to many people's surprise, is not part of the criminal law. (There's a good CBC backgrounder here.) Hence, you can't be arrested for not obeying such a piece of legislation. It is, on its own, completely toothless. It's more a PR thing than anything else: "look! look how we're trying to protect the poor, innocent hoi polloi!" If back to work legislation is defied, then someone has to go to court and get an enforcement order. Which can also be defied. You can't be arrested for breaking an enforcement order. However, if you break an enforcement order, and someone goes again to court to complain about it, then someone -- usually the union leadership -- can be arrested for contempt of court. But, unions have pretty good lawyers, typically, so don't expect the leaders to spend much time in jail before trial.
On the whole, it's a stupid idea. If you have a genuinely pissed-off union -- which is hard to gauge in this case [applies to the TTC and York U strike], but since the membership has broken with the negotiating team in a fairly significant way and they struck in such a way that people were left stranded, I expect they are [this part, of course, is TTC-exclusive] -- they will just flip off the government and keep striking. And there's a number of hoops that would have to be cleared in order to punish them for doing just that. And even then, they may decide the punishment is worth is [sic]. All back to work legislation in such a circumstance really accomplishes is causing both sides to dig in their heels and prolong the dispute.