A little bit about drag-free drift.

Drag-free drift
Presenting the fly in a natural manner is the fundamental definition of drag-free drift. This means that when the fly is floating naturally through the water it moves with the currents at a same pace as the other food sources and particles. The fly line, leader, tippet and fly have some measure of physical resistance against the water. Resistance is magnified when the fly line, leader, and tippet are pulled “Dragged” through the water by certain factors, primarily the downstream current. Drag is reduced by providing slack in the system; fly line, leader, and tippet. A combination of proper casting, mending fly line, leader construction and tippet length is how slack is introduced. This should not be confused with eliminating slack for efficient casting. The slack explained here, for drag-free drifts, is introduced after the cast is made, either in the air after the final stopping motion of the rod tip, or on the water by utilization of mending fly line; typically upstream.