What does this part of the fuse box do?

I have a 2012 Prius II and one day heading to school, I lost power steering and power breaking after getting of f the freeway. My mechanic checked it and the previous owner soldered a cable to the white cable in the diagram connecting to the part I circled in the photo. Does anyone know what it is or what’s it called I’ve been looking online at diagrams and can’t find anything.