(Reuters) - Boeing ( BA ) faces a likely strike on Friday by more than 30,000 workers that could shutter its Seattle-area plane factories over demands for a bigger pay increase than the 25% over four years that union leaders agreed to.

If there is no strike, the unionized workers could have to swallow a contract that most of them have said they oppose. That is because they need a supermajority to strike under rules from the International Association of Machinists and Aerospace Workers (IAM), the largest aerospace union in North America which represents Boeing's ( BA ) U.S. West coast workers.

Here's how the voting works:

Q. What's on the ballot?

A. Boeing ( BA ) workers have a two-part ballot. They are first asked to accept or reject the contract offer. The second question is whether they want to support a strike.

If a majority of workers vote to accept the proposed contract, the agreement goes into effect.

Q. What needs to happen for the workers to go on strike?

A. A majority of Boeing ( BA ) workers must reject the proposed contract and two-thirds of those voting must support a strike.

Q. What happens if a majority of workers reject the tentative agreement but less than two-thirds of those voting support a strike?

A. The contract offer is accepted by default.

Q. In general, how often are contracts ratified despite support from just a minority of workers?

A. Harry Katz, a professor of collective bargaining at Cornell University's School of Industrial and Labor Relations, said such a scenario is uncommon. In one rare case from 2002, IAM workers at a Boeing ( BA ) plant in Wichita, Kansas were forced to accept a contract with less than 40% support. That's because only 61% voted to authorize a strike, falling short of the two-thirds majority required under the IAM's constitution.

Q: Why does the IAM have such a high bar to call a strike?

A: The IAM says that if a simple majority was enough to call a strike, the chances of winning a good contract through a strike "decreases dramatically."

"Anytime you go into a possible strike situation, you want to be sure that a 2/3 majority of your membership support calling a strike. Going on strike with less than 2/3 results in a possible weak picket line," according to an IAM information package for workers about the Boeing ( BA ) talks.

"Calling a strike is serious business. The leadership needs to be sure that 2/3 of the membership support withholding their labor to walk the line."

Q. Do workers get strike pay?

A. Members will be paid $250 a week from the strike fund starting in the third week, a union spokesperson said.

Q. When does the voting start and end, and when will we know the result?

A. Polling started on Thursday at 5 a.m. PT and will close at 6 p.m. PT. The result will be announced on Thursday evening, the IAM said. If a strike is sanctioned, it could start early Friday.