Do not forget that in the early 90's there was a helicopter on a rig to rig flight, of just minutes, that turned downwind at night (I think) after departing a North Sea oil rig. All perished. Verdict was the loss of performance during the turn downwind. Relative speeds are important. It blows its arse off in the North Sea and these guys were probably in visual contact at night (at what may have been 4 pm) with their destination in a helicopter that really performs.
It's a real phenomena. Don't underestimate it in slow flying/underpowered aircraft or in low level/ low visibilty conditions.