What tactics can unions and employers use to pressure a contract settlement?

"Virtually all labor agreements specify peaceful resolution of conflicts, usually through arbitration. However, when a contract expires and a new agreement has not been reached, the union is free to strike or engage in other efforts to exert economic pressure on the employer. A strike occurs when employees refuse to work. The United Auto Workers union used a selective strike strategy, a strategy of conducting a strike at a critical plant that supplies parts to other plants, against General Motors. The union conducted its strike at a stamping and parts facility in Flint, Michigan, that supplied critical parts to other plants. The 54-day strike caused the company to stop production at many of its assembly plants because parts were not available from the Flint plant. General Motors lost approximately $2.2 billion during that dispute. Likewise, the employer can put pressure on the union through a lockout or by hiring strike replacements if the union has called a strike. For example, in 2018 aluminum producer Alcoa locked out more than 1,000 union workers from its smelter facility."