The Combined method is a knitting technique developed by Annie Modesitt, author of Confessions of a Knitting Heretic. In reality, this method is a modification of existing knitting methods, and hence is used in conjuction with either the English Method or the Continental method.
So, what makes the combined method different? Well, the Knit stitch in the combined method is created by performing a Knit through back loop. Meanwhile, the Purl stitch is created by looping the yarn in the opposite direction one normally would, thus creating a Twisted stitch. Experienced knitters will note that one would Knit through back loop in order to fix a twisted Purl stitch, and thus the end result is the same as standard knitting (i.e., the stitches are still set up the same way).
The combined method generally results in much more even Gauge. In fact, Annie Modesitt claims her pieces are often compared to machine knitted pieces. Moreover, she claims this technique, when used in conjuction with the Continental method is easier on the hands, and is generally faster.
The down side? Some patterns may need to be converted, due to the differences in the way the stitches are set up.