This immature female specimen is moulting. The old skin has split open at the back, behind the head. It squeezes itself out of the opening, while it unfolds its new, bigger skin. This specimen will moult two more times until it is fully grown. It has somehow (probably in an earlier moult) lost its left hind leg, as you can see in the picture: with only two legs it hangs on to this side of the bramble's stem.
In the following pictures you can see that the bigger, renewed body has a rudimentary new hind leg, an example of regeneration common in stick-insects.