Grayline Operations is when you are able to use to your advantage the reaction of the D-Layer to Sunrise/Sunset. During Dawn and Dusk the D-Layer changes are done slowly. At that specific time a Station Sitting on the Grayline (virtual line seperating Day from Nigth!) could use this to his advantage.
Transmiting a signal from the Grayline toward the night side of the earth migth actualy gets reflected by the D-Layer (seen in Blue in picture below) at a angle such that the F-Layer (seen in RED in the picture below!)would not absorb the transmition. Thus the signal would then be reflected down to earth and back again in a similar path on the other side of earth!
Grayline operations occurs mostly on 160M, 80M, 40M and 10M occasionnaly we may come accross some Grayline operations on 15M and 20M.
PA3CQR came up with a fairly good program to evalute Grayline Propagation based on Algorithm from ON4HW.