That's what the bending rules are for. The ring itself is metal. The zone lines are water grates, and these [ he points to the smaller circles dotted on the diagram ] are clay discs. An earthbender can only use the discs in his zone, and a waterbender can only use water from the the grates immediately in front of or behind him. Firebenders have a bit of freedom because we generate our own element, but there's rules about the actual attacks themselves for all three elements. No head strikes with earth or fire, no water or fire blasts longer than one second. No ice or steam, no breaking discs before they're thrown, and no bending metal.

[ NEXT UP, TIE BREAKERS. DA DA DA DADAAAA ]