I invite you to talk in this site about Remote pairs, a particular case of XY chain, my essay from page http://www.sudoku.org.uk/discus/messages/2/3507.html?1171195067.
Sudoku.org.uk forum has been the only one in which I contrive to attach my essay as a word document, in initial form. Also, Andrew Stuart, the godfather of Remote pairs technique, has been so kind to place this essay on http://www.scanraid.com/AdvanStrategies.htm#RP
and http://www.sudoku.org.uk/sudokulinks.asp web sites. You will find a series of exciting exercises, which are waiting for response.
My study needs as well some observations and addings. Bob Hanson has sent me a pertinent alternative method to resolve the grid from Figure 3 with Forcing chain algorithm.
Good luck!
Mihail Iusut, Romania