This cast on is basically a series of slip knots placed on your needle. It creates a very elastic edge and is great for the cuffs of socks, mittens, and sleeves plus anywhere else where you want a very stretchy edge. It works especially well with ribbing and adding stitches at the end of a row.
One important point when doing this cast on: you need to get the stitches snugged up close to each other – avoid gaps between the stitches as it will give an untidy looking edge.
Link to video: