The Thundering Wind Hammer is a special and difficult kung fu technique dubbed as one of the Twelve Impossible Moves. It was seen in use by Po in the episode "Owl Be Back" from Kung Fu Panda: Legends of Awesomeness to defend the Jade Palace against Temutai and eventually Fenghuang, the last known user of the technique.
As demonstrated by Po and Fenghuang in "Owl Be Back", the Thundering Wind Hammer is done by rapidly spinning around, suddenly stopping, and then placing one's palm out towards the target. A ball of energy shoots out from the palm and flies toward the target, blowing it back on impact.
The only known way to take a direct hit from this energy and not get harmed is to deflect it, also demonstrated by Po and Fenghuang in the episode. This was done by using one's arms (or wings, in the case of Fenghuang) to divide the approaching energy and pushing it away.
In "Owl Be Back", Po used the technique to send Temutai flying from the Jade Palace after he and his soldiers invaded. After seeing Po use this powerful technique, Shifu and the Furious Five began to fear that Po would eventually turn evil, like Fenghuang.
When Fenghuang returned to the palace later in the episode, Po attempted to use the technique to defeat her, but was shocked when Fenghuang deflected it. Fenghuang explained that you deflect what you can't absorb, and then used the same technique on Po, but Po easily copied her moves and deflected it himself, much to her pleasure.