Faster casting changes three things — bend, force and pause — and the loop should look the same. The examiner wants obvious separation between the speeds, so your slow has to be genuinely slow.

What the instructor says

• Three things change. “You’re going to get a bit more rod bend as you go from slow to medium to fast”, so the casting arc increases just a little; more force and acceleration; and a shorter pause. Those are “the three things the examiner will ask you about.”
• Stroke, not wrist. More stroke length, not wrist — think pull, pull, pull — and start smooth, or “you’re going to get a tailing loop.”
• Go as slow as you can. “The slower you make that first false cast, the easier it is to get fast.”
• Expect any order. Fast, medium, slow; or medium, slow, fast; he may have you call the change or direct it.

Putting it to work

1. Start wide, close it down. Begin the slow loops wider than you can make them, then shrink them as you speed up: they end up about the same size.
2. Match the speed to the water. Slow for a delicate presentation, fast to get under an obstruction or into the wind; instructing, as slow as you can with good loops.
3. Make slow really slow. If the first false cast comes out medium, there is nothing left to show when he says fast.

Source

From the FFI CI walkthrough series by Dayle Mazzarella (Chapter 3 — FFI Casting Instructor Certification Tasks, Fly Fishing Coach International), lesson 8, 00:00:29. See the video library and the task pages for the exam wording; terms follow the FFI Fly Casting Definitions.