Laws about labor strikes

From what I’ve heard in the news, the contract for United Airlines’ mechanics and technicians has expired, so they no longer have a contract. I’ve also heard that the union members need the permission of the courts before they can refuse to work overtime or strike.

If they don’t have a contract, why are they required to work overtime?

Also, what has to be accomplished before they’re allowed to strike?

Are other workers subject to these sorts of restrictions before they can refuse to work overtime or strike?
My apologies if I’ve misremembered any of the details of the labor dispute.

Two separate (but related) things are going on, here.

A specific contract is only one aspect of the relationship between a union and a company. The contract covers the provisions of remuneration, benefits, work rules, etc. for a specific period of time. However, the management/union relationship is not defined strictly by each contract. There is also the certification that occurs to identify a specific union as the representative of the workers. That certification carries its own set of rules. The workers may dump (decertify) a union and either certify with a different organization or go non-union, but the company cannot simply dump the union. Thus, when a contract has expired, the company and the union are still bound by the certification process to continue to work together (with however much animosity). If a company feels that a work stoppage will seriously damage it, they may petition the courts to order the union to continue working.

In addition, under one or more of the various laws written to protect the public welfare from being disrupted by shutdowns in the transportation industry (especially The Railway Labor Act invoked in this case), a union may be ordered to continue working.

If the union chooses to ignore the court order, the union and each member may be fined daily for as long as the work stoppage continues.

Under these conditions, the court may order the union to continue working under its old contract (with or without some modifications) in order to maintain its certification. The Railway Labor Act gives the courts more power than even the general nature of certification.