A flexible steel wire with a number of magnesium anodes along its length. Rather like a string of sausages. This has a rigid section at one end with a threaded connection. 

Flexible aluminum and magnesium anode rods are designed for those limited spaces where a straight anode rod will be unable to be inserted into the water heater due to limited access directly above, such as low ceilings, closets, attics, under stairwells or in basements. Just bend the anode rod at its flexible sections and insert into opening at top of water heater one section at a time until all sections are inserted. Just make sure that the anode rod is not touching the side of the water heater once completely inside.