Playing Fair.
Whenever they wrestled together, Kun knew he could easily use his earthbending skills to make a fool out of Sokka at any time. Even when the Water Tribe youth, in the heat of the moment, tried to goad him into using his full capabilities, Kun still held back anyhow.
But he also did enjoy hearing Toph or Suki mention to him later on how sweet it was of him to let Snoozles win his share of matches too.
For his part, Sokka never willingly took advantage of his opponent's crippled leg.