As you look at the first stitch on the left hand needle (LHN) you will notice that it is a loop that has one half of the loop in front of the needle and one half behind the needle. Usually you knit by going into the stitch moving the right hand needle RHN) into the stitch from left to right, in front of the needle. To knit through the back means to knit into the back loop of the stitch. To do that have the RHN approach head on into the first stitch (like you would to purl, moving from right to left) but behind the LHN. After you insert the RHN work the rest of the stitch like any knit stitch.