A couple of people have already made reference to it, Wirbelstrum being one of them. I have since trawled my manuals and can only find one reference in the BFCTM. I thought the info I was looking for was in the FCOM Automatics! I am sure there is some logic along the lines of.......if you are less than 5' for more or less than 2 seconds one condition will not give you FD guidance and the other will not give you thrust, requiring manual input.....buggered if I can find the reference!
Regards
GBD