The screw accessed via the trigger guard adjusts pull weight. “+ ABZUGS” = heavier. “- SHRAUBE” = lighter.
Problem is that with use, because of their cheap laminated sheet-metal construction, BSF triggers wear with use, becoming unsafe. At which point you need to wind in the direction of “+ ABZUGS” and function test to assure safe operation.
Always function test a newly-acquired one immediately in a safe way to see if it comes with a problem. Load, cock, safe direction/backstop, rubber mallet strikes on the butt, that kind of thing.
For the rest, stripping and cleaning off aged grease and re-lubing is never a bad idea. I’d also say very light polishing, but be really really careful for the reason above.
There may be another level above this of BSF trigger-fettling, but if there is I have yet to hear of it.