Shuffling Along and Cycling Around

Many readers will at some time have played games with a pack of cards.  In most games one begins by shuffling the cards so as to randomise their order.  There are various different ways of shuffling, one of the most popular being the riffle shuffle.  In this shuffle, the pack is split into two roughly equal parts, one is held in each hand, with the edges adjacent and the thumbs are used to flip through the two parts so that they are merged together again, cards falling more or less from each hand alternately.  If you haven't seen a riffle shuffle before, this description probably didn't help much!