Here's the relevant text:

"Places a large, stationary Anti-Magic Zone..."
http://www.wowhead.com/?spell=51052
With some math, the probability of cleansing on a given pulse with 2-3 totems is actually MUCH better. Pr(failure) = 9% for two totems, 2.7% for three. Since we're talking dots, they can get one tick, sometimes two. Possibly annoying way to spend talent points vs Paladins or what not, but seems very useless versus shamans. Still, it's a good talent to take anyway - it also gives % hit.