When knitting top-down socks with a heel flap, you will need to pick up sts from the edges of the flap to form the foot part of the sock. To enable picking up of these sts, it's easiest to make a so-called "chain edge" on the two sides of the flap.
There are several ways of making a chain edge. Montse Stanley's book has at least 3 methods (of which one is actually incorrect, in my copy). The usual method involves doing the same thing on both sides of the knitting, usually something like: Sl 1st st PW, knit last st. Alternatives are Sl 1st st KW, P last st, or slipping the last sts and always knitting, or purling, or KTBL'ing the first st.
But I find that all these methods lead to one side of the flap having a very loose and ugly chain edge, while the other side is nice and tight. After experimentation, I think this is because I usually do slipped-stitch heel flaps, for reinforcement. Those slipped stitches only happen on one side of the flap.
I've found that I get a better edge if I also Sl the 2nd last st on the side of the flap that isn't being slipped. That slipped stitch means that there is less yarn present for the chain edge, which is then tightened. So here's my recipe for tight chain edges on both sides while doing a sl-st heel flap.
The selvedge takes up the first and last two stitches of each row. In between you can do whatever you want for the flap (ex. "eye of the partridge", slipped stitch ridges).
RS: Sl 1 PW wyif, K1, {do whatever you want to last 2 sts}, Sl 1 KW, K last st
WS: Sl 1 PW wyif, P1, {do whatever you want to last 2 sts}, Sl 1 PW wyif, K last st
...repeat these rows for the flap.
geeky aside: The 2nd last slipped st is slipped in such a way as to keep the slipped yarn in the back of the flap. You could slip it the other way, moving the yarn to the opposite side, to make it more exactly symmetric with what's happening on the other side of the flap...but then you'll see the little bar of the slipped st at the edge. Your choice.
If you do a straight stockinette flap with no slipped stitches, or linen stitch, where both sides are slipped, then this technique has little benefit.
