not the link i remembered but some good info:
http://blog.zorinaq.com/?e=42It is not very well known, but the PCIe specification electrically supports down-plugging a card in a smaller link connector, such as an x16 card in an x1 connector.
However PCIe mechanically prevents down-plugging by closing the end of the connector, for a reason that I do not understand.
Some motherboards require pins A1 and B17 to be shorted for an x16 card to work in an x1 connector.