to unlock this you need to make a block at the net, the icon for blocking is a pair of hands. when you see it you need to jump and make a similar movement you would to perform a spike, but you need to react very quickly, if you don't you'll almost always miss, it took me four or five attempts to nail it.
This doesn't seem to happen on the easier diffculties, but if you turn it up to proffesional you will get plenty of chances to make a block.