The phrase "safety valve" refers to something that allows for the release or relief of pressure or tension in a situation, avoiding a more serious or dangerous outcome. Full definition
When women in some countries have more than six children, while others have barely more than one, trading people is an obvious safety valve for both sides. (scientificamerican.com)
Running backs saw one target for about every three carries in 2014, and they are frequently used as safety valves. (sbnation.com)
This would act as a democratic safety valve giving a new party a voice in parliament, but would be no substitute for winning constituency contests. (dprvoting.org)