Leroy is correct. The candidate is asked to qualify the line by making a series of switch casts (5) along a measured line. Since it may be difficult to accurately measure the casting distance when a change-of-direction is added (such as a single spey), we first determine whether the candidate and candidate's line can meet the standard with a switch cast, then go through the fishing casts. When I assisted Simon with his test, we also did the same thing with the 100ft cast.
Throughout the test if there is ever a question about the casting distance (for example, a leader that doesn't straighten out, or a candidate who pulls the rod tip up at the end of a shooting cast), the candidate is asked to cast again.
When preparing for the test, I believe it is vital to remember that it is a pressure situation no matter how relaxed the candidate and/or examiners are. To consistently meet the distance standards, the candidate should be one who can consistently exceed the standards during practice. Someone who cannot cast 110ft+ cleanly and consistently during practice could easily run into trouble during a test if they freeze up. Also, the test can take 3 - 4hrs to complete, so fatigue becomes a significant factor. While it is not necessary to have tournament casting skills to pass the THCI, a candidate should possess well above average casting skills, particularly the ability to quickly analyse their own casting and adjust when necessary.