Same as magnetic chuck, you get much better holding force with multiple alternating strips instead of one two huge poles. Also the way they are swithced on and off is really nice, see https://en.wikipedia.org/wiki/Magnetic_base and http://s3.amazonaws.com/product-database/images/2/AX47-P_how_it_works_original.jpg but in the second one i think the poles should be alternating.