What is Nesta doing when Lucien observes her during a training session, and what is Lucien's reaction?

Lucien arrives while Nesta is practicing her punches against a padded punching block. He watches as Nesta punches the padded wood over and over again. He sees her hit the wood so hard that it splinters. When she leaves, there is a burn mark on the punching block and the wood glows. Lucien is concerned. He tells Cassian, "Mother spare you all" (419).