Normally these ageeements are time limited to say 1-3 years and then needs to be renegotiated, this is when union requires some certain salary increases over the next agreement period and for that promise to not strike given that the employer hold up the agreement. Should the not come to a agreement during the negotiations, strikes are an option.
[1] https://www.malarenergi.se/om-malarenergi/framtidens-samhall... [2] https://www.svt.se/nyheter/lokalt/ost/de-lagrar-varme-i-berg...