During trials in Kenya and Morocco it was discovered that if you conduct a DL with the door fully open then the dust cloud will start to encompass the aircraft from the vicinity of the RHS pilots window. With the door half way open the the cloud starts in the vicinity of the forward starboard window. With the door two thirds open then the cloud starts just forward of the door.
The Boffins put it down to re-circ. A little sceptical at first the theory was tried and blow me if it wasn't correct..... Therefore....best practice was to have the door somewhere between two thirds and halfway open....In essence it gives the handling pilot about another 10 seconds of viz before he is in the lap of the gods.
The same re-circ issue was trialled on a Type 23 Frigate....same details same result.