Most knots (clinch, palomar) tighten directly against the hook eye, locking the fly's nose to the line. For dries and nymphs, that's fine — the fly doesn't need to swing. For streamers and articulated patterns, the Loop Knot creates a small open loop at the eye, letting the fly pivot and waggle naturally as you strip.
The non-slip mono loop (sometimes called the Kreh Loop after Lefty Kreh) is the most common variant. Tie an overhand knot in the standing line, pass the tag through the hook eye and back through the overhand, wrap the tag around the standing line 4–5 times, then thread the tag back through the original overhand loop. Wet, tighten slowly.
The loop should be small — about 1/8 inch. Too big and cast turnover suffers.