Here are some pictures you can have a look at www.pindiy.com/forum.php?mod=viewthread&tid=402790
Im a beginner, doing a lot of research at the moment, havent done done anything with yarn under. Must try :)
For doing certain stitches, which I am unable to come up with a name right now.... it is important. Criss-cross stitch maybe??? I learned to do yarn over and do it for everything unless a certain stitch is called for which required yarn under.
I prefer yarn over, but for some patterns with colour changes on the amigurumi I use yarn under because it creates a straight line if you do it for several rows
I prefer YO but it also depends of the yarn, the piece and the texture of the fabric I want to get finally. YU is tiring as it require too much effort to do.