Why Did Master Rinzai Slap Jo?

In the Blue Cliff Record, Case 32, a monk named Jo asks Master Rinzai, "What is the essence of Buddhism?" In response, Rinzai grabs the monk and slaps him. Why?

In this talk I explain what all this hitting and slapping is about in Zen. There's a lesson in this story, about sustaining our own meditation practice over the long haul.