Use a pile cast when you want slack line on the water produced by the cast itself — most obviously when fishing downstream, where the heap of line lets the fly drift without drag.

• A pile cast is one way to produce a slack line presentation; slack line reduces drag on the fly.
• Do not lower the rod tip too quickly after the stop, or the cast becomes a pile mend.
• Practise narrow and wide loops — wide loops can produce more slack.
• Keep the delivery between roughly 20 ft and 30 ft: aimed too high, the fly lands closer than 20 ft; too flat, and it lands beyond 30 ft.

It is only one option, and it can be problematical in wind, where ticking on the back cast and loss of control are the risks. In wind, a bounce cast or a ‘high stick finish’ puts slack in the line instead.

Source

From the CI Study Guide (2025) (PDF), Task 5 — a cast that produces a slack line presentation; terms from the FFI Fly Casting Definitions (Slack Line Presentations).