To access the data on a strip, the carousel rotated to bring the required strip under the access mechanism, which would then extract the strip and wind it onto a small drum. The drum had 20 read/write heads positioned next to it, which could record and access data on the magnetic strip as the drum rotated. The set of heads could also move parallel to the drums' axis to one of 5 positions. This then gave 5 sets of 20 tracks on each strip, and each track held 2000 bytes. 
The carousel was composed of 10 cells, any one of which could be removed and another inserted in its place. The strips lived in subcells of 10 strips, and there were 20 subcells to each removable cell. 
The total capacity therefore was 10*20*10*5x20x2000=400 million bytes.

And it was fast. It could whip the current strip off the drum, and replace it with any other one in the carousel in typically half a second!  Must have been fascinating to watch.

It may seem that this contraption is overly complex, but the only other random access, removable media device that IBM had at the time was the 2311 disk storage drive, which stored only 7.25 million bytes per disk pack. For 55 times the capacity, but only 7 times as slow, they must have thought it worth the trouble.