It is an extemely energetic interpretation of Tschaikovsky's popular dance from the Nutcracker. The dance itself is in Ukrainian and was referred to, by Tschaikovsky himself, as a "Little-Russian" (i.e. Ukrainian) piece.
Yaroslav brought down the house as the Trepak, the Ukrainian character who bounds onstage, whirls, leaps, kicks and otherwise astounds; Yaroslav danced it for all it was worth.