I understand what you're saying.
When you knit with the main color across your motif, you slip the cc's. When you purl back you only do the cc's, so the mc is where you left it and the cc is where it needs to be for the next round. Then you slip it all back over so the working strand of the mc is where it needs to be for you to start knitting. It really is hard to picture if you haven't done it.
The only way I can picture it working is when the mc is part of the motif and it gets carried across on the "first pass". Then you're going backwards for only the cc, then coming back, through slipping, for the mc where you left it on the left side of the motif after you knit that color across. :?? I'd probably be just as clear if I wrote in Finnish! I've done it, and could show you quite easily, but maybe if you sit down with it you'll be able to make it make more sense.
If not, I'll be here all evening, come on over!