The speed of the current just inches below the stream surface is usually moving faster than the current on the surface where our dry flies drift. When the leader tippet is pulled by this fast current it can produce a subtle drag on the fly which may be difficult for us to discern, however, the trout quickly detect this and may refuse to take our fly. In order to prevent this drag I always dress my dry fly leaders frequently with dry fly cream floatant. This keeps the leader on the surface where it drifts at the same rate as the fly. This produces a natural drifting dry fly and the trout take it quickly.