In reviewing the entry for the Quickstep Fishtail in The Ballroom Technique, we find what may very well be a typo. The notes for this figure state that the man's 3rd step should be slightly outside the lady's left foot. To set context, the first two steps are RF in CBMP, outside partner, and LF crosses behind RF. So we are in outside partner position going into step 3.
At this point, if we have good connection, and we are in outside partner position, stepping outside the lady's LF requires at minimum that the man swing his leg out and around the lady's right foot.
However, if we read that note to state that the man's 3rd step is slightly outside the lady's RF, then the movement is natural.