aka Mid-Sealed Socks.
I've been an advocate of Porelle Drys for some time and agree with lots of other MTBers that repeated immersion allows ingress of water through the socks one design fault - the hole your foot goes in! Even with the calf length version.
SealSkinz claim that with Waterblockers this doesn't happen.
The sock is an oversock, i.e. a liner must be worn. It has a coolmax liner built in as well to aid breathability. The seepage is prevented by way of a membrane about 3/4 of he way up the sock and by an elasticated cuff that folds down over the membrane part of the sock, to aid contact with the skin.
After a 3 hour plus ride through waterlogged meadows, hub deep puddles, bloody mud everywhere and one unfortunate incident with a stream, the only conclusion I have is these are the best thing since sliced bread, no bread itself! My feet were bone dry and thanks to a liner sock - warm. Not bad considering I was wearing Answer Impacts. Although they retail at £29.99, for really wet days, everyone should have a pair.