I think Renato meant to write 12 seconds instead of 21 seconds. Here is one description of the workout from Renato from another post awhile back:
I remember one training from Said Aouita, that I saw in 1984 in Tirrenia before winning Olympic Games in LA :
3 times 1200m alternating 150m in about 18.5 / 19 and 50m without pushing, in about 10 / 12 sec., with final times between 3:02 and 3:04, recovering 5 min.
I know that this type of training was alreday used by Alberto Juantorena, preparing Montreal 1976, when he was able winning 400 and 800 in the same Olympic.