Dmitry Komarov (aka DBK here in forum) explains it here: http://www.cues.omsk.ru/school.htm and here: http://www.cues.omsk.ru/school2.htm
Even if you don't understand russian, pics should be enough to get it.
Here you can see some blanks: http://forums.azbilliards.com/showthread.php?t=182680
And...