Yeah Rijkaard was brilliant in that system. As I remember it he sat in front of the back three. Dropped into defence when needed but stepped up into midfield when Ajax had the ball. As you say they could change their shape transisitioning from defence to attack.Also their system was peculiar because at the time Rijkaard was used to play as a CB and Reiziger/F. De Boer were initially fullbacks, so Ajax had the ability to completely change their structure when needed.