Your question was asked recently on another website.
The response was to go to the following address and download the AIRBUS PDF "Getting To Grips With Aircraft Performance".
Getting to Grips With Aircraft Performance
I do not know if you were the originator of the question in both websites, but the similarity in the wording in the two sites suggests that this might be the case.
Before composing this message I went to the other website, followed the link and downloaded the AIRBUS PDF. Although it takes quite a while to download, it really is a very good book. If certainly includes quite a good description of drift-down.
I suggest that you download the AIRBUS document and read the section of drift-down. If you still have any more specific questions on the subject you can then post them here, and hopefully get a suitable explanation.