This website describes the relation between Visa and Mastercard (short answer: they are competitors, which provide virtually identical products).
http://wiki.answers.com/Q/What_is_the_d ... Visa#page8
The really bizarre behaviour occurred when I tried to back up. Violent left-to-right-and-back oscillations, lasting a long time, eventually calming down and evolving into mild tremors.
Does anyone else get this? I've never seen anything like it.