Every higher pet is given a new procc ability (stun, ice-dd,life-tap,bubble). The general chance to start an ability depends on a swing on target. Each ability should have his own chance (maybe 5%-10% i dont know) to procc on his own, so that later pets are able to procc multiple proccs at once with just one swing. Not seen yet on my pet, seems only to procc at least 1 procc with 1 swing.