I'm not too knowledgeable in sock making, as demonstrated by a pair I've been wrestling with since April. What makes a heel French?
The French heel is one of many technique on how the gusset of the heel is made. The stitches are held on a stitch holder and are worked later. Look through some learning how to knit (or how to made socks) on the many gusset of a sock heel is made. If you want the instructions on how to made my socks, just let me know and I will e-mail them to you.