Well they’re not really well enough hidden to call them secrets. But basically in recent times I’ve been getting into some very complex maps. I’ve encountered situations that are just to hard to address in the mapper so have resorted to using XSLT. So basically I write my XSLT then I set the "Custom XSL Path" property on the BizTalk map to override the autogenerated XSLT.
From time to time I come across a situation that I find easier to deliver in the mapper. So I’ll delete the "Custom XSLT Path" value in my map, use the mapper to create the solution, do a validate map, then ctrl+click the generated source from the Visual Studio output window. It’s a great way to learn XSLT syntax actually. You can also view how some of the basic functoids work aswell. So even though I’m bypassing the mapper for some solutions the mapper is still providing productivity benifits which is after all its main intention.