Here's a couple of websites discussing:
https://museumofprotest.org/methods/slowdown-strike/
https://www.tactics.nonviolenceinternational.net/tactics/Slowdown%20strikes
https://archive.iww.org/about/solidarityunionism/directaction/1/
It basically means we reduce output across the board. For instance, if I work in a data center, I fix 5 servers per day instead of 10. If I work in an ER, I work as usually for emergent patients, but for non-urgent patients I reduce how many I can treat per hour. It has been used previously as a precursor to a strike or as a stand alone action. In 2015, the longshoremen union had a 4 month showdown resulting in $2 billion in losses per day
We could expand the slow down outside of the workplace as well. Like we all use the self checkout slower, drive slower, eat slower at restaurants (esp national chains w/out waiters)