So, for them and for those of you who thought the same thing, but didn't comment, I thought I'd better explain how to make this type of hem as best as I can.
There are probably a lot of other tutorials for this out there, and better ones as well, but I don't know them. So, to answer any questions, I'll just show you what I do.
In fact, I've only been using this technique for a little while. Before, I sort of knew about it, but it wasn't until my boyfriend bought a couple of RTW t-shirts with this type of finish that I was interested enough to try it out.
It all starts, as so much in sewing, with pressing. Press the amount of fabric you want as the hem up at the wrong side of the fabric (nothing special there, you'd do that for any kind of hem).
The second bit of pressing is a bit more unusual: Press you hem to the right side of the garment along the line at which the raw edge has come to lie after the first pressing.
Now, it's time to get the serger ready. Normally, when making a garment in jersey, you'll be using two needles and four threads on your serger. For flatlocking, you need one needle (the one on the right) and three threads. The tension settings which work well on mine are: a normal setting, 4, on the upper and lower looper and a reduced tension, 2 (normal would be 4 as well) on the needle.
And then you can start hemming. carefully guide the edge which has both a fold and the raw edge betweenthe needle and the blade. You don't want to cut anthing away. In fact, try to keep a millimeter or so between the serger blade and the edge of the fabric. (of course you could swivel the blade away but I don't. Having it in the normal position makes aiming easier IMHO).
When the serging is done, the hem will look like this (on the wrong side of the fabric).
Fold it open (on the right side) and pull a bit to open the stitches.