1. Straight-line rod tip path. The tip travels a straight line through the stroke, so the fly leg is straight and the loop narrow. Unchanged from 30 to 50 ft: it results from the other four.
2. The casting arc varies with the length of line past the rod tip. At 50 ft the rod bends more, so the arc — the angle change of the rod butt during the casting stroke — must widen to keep the tip path straight. That is Task 21’s required change.
3. Proper force, applied smoothly. More line needs more force, or the same force over a longer distance; lengthen the stroke rather than overpowering the cast.
4. Slack line kept to an absolute minimum. Start every stroke with the line as straight as you can; slack costs more at 50 ft.
5. Pause varies with the line outside the tip and the line speed. The 50 ft loop takes longer to unroll, so the pause lengthens to let it straighten before the next stroke.

Alongside the five, keep line and intended rod tip path aligned with the target (the 180-degree principle), and haul as the carry reaches 50 ft.

Source

From the CI Study Guide (2025) (PDF) — the Five Essentials and Task 21; terms from the FFI Fly Casting Definitions.