I saw that clip of Ed and another angler discussing the origin and merits of the intruder style a year or two ago. Since then I have been giving each new twist on the style the swim test before I commit to a larger batch.

The action in these flies comes from the ostrich and the rhea. When I first started tying them, I was using longer waddington shanks (45mm) which gave me a fly in the length I wanted, but they didn't swim all that well. More recently I have been using shorter 25mm-35mm shanks while leaving the materials the same length. These flies swim much better. I think one of the key concepts with the intruder is to give the appearance of bulk and mass without actually achieving a bulky fly in the process. Flaring the materials and using fairly stout materials for the collars and underwing features really helps with that.

Leaving the body of these flies pretty sparse help with a number of things. First, a bulked out body doesn't get in the way of the "action" of the fly, letting them move freely and giving a more lifelike appearance. Keeping the fly more on the sparse side lends to a more castable fly that gets into the zone more readily as well.

The test I give them is to dip them in a glass bowl and swish em around a bit. Not exactly scientific, but if they are pretty wiggly, I keep em, if they don't move much, I take them apart and start over.