Some patterns specify "slip 1 purlwise" so it does become confusing when a pattern simply says "slip 1".
I would interpret (k2,sl1) as k2, slip 1 purlwise with yarn in back and (k2 sl1 wyif) as k2, slip 1 purlwise with yarn in front. Check the Glossary tab at the top of the page.
When slipping purlwise, you don't always hold the yarn in front though. It usually stays in the back if there's knit sts before or after it. Unless it says to like this pattern. Purlwise or knitwise refers to how the needle is inserted into the st, but not how the yarn is held.