The Pile cast, along with a few others, is a cast belonging to the family of slack line casts that we often require to combat differing currents and obstacles whilst fishing. One particular way in which it can be used is to give your fly a very long drift before drag is imparted due to the direction of your cast in relation to the flow of the current e.g. when casting downstream, but it also has other uses.
For this particular application, simply make an Overhead cast with the introduction of a very low back cast. This allows you to make a very high forward cast, thus making your fly climb in the forward delivery. As the fly is projected forward, and upward, simply drop the rod tip onto the surface of the water in front of you, this will inevitably bring your fly line down before your fly, therefore creating a pile of slack line on the water in front of you prior to your fly touching down. This in turn will allow your fly to float freely downstream for a period of time before your fly line straightens and introduces drag once again, at which time you will need to re-cast for another drag free drift. This technique is especially good if you want a fly to drift naturally downstream under overhanging branches where fish will inevitably lay. Look out for slack line though when striking a fish and be ready to compensate for this with what might be a much longer strike than normal.
This technique along with the Wiggle cast can be used effectively with several types of flies, although it does work particularly well with dries.