It happened off camera in the broadcast. I doubt they got it on another camera either since they did not go back to a replay on the incident.
It happend as the Sharks were transitioning, and Johnsson was holding JT's stick through much of the neutral zone (which was not called) right before which was all we saw.
That was precursor. JT got tossed from the face off for what I thought was no good reason other than he completely dominated the Lighting in FOWs.
Just watched the play. Starts with 16 seconds left in the 2nd period. The puck is in Lightning end and is moving out. Thornton is skating after the puck and Tyler Johnson clear as day hooks and interferes with Thornton who is probably still pissed from 1. getting tossed from faceoff and 2. not getting any penalty calls for the second game in a row on blatant penalties. Thornton then interferes with Johnson as they go through the neutral zone a little bit and Johnson responds by grabbing Thornton's stick. After they enter the Sharks' zone, with 8 seconds remaining in the period, Thornton slashes Johnson hard on the back of the leg (probably still thinking the refs aren't calling shit) and follows up with a cross check to Johnson's head.
Thornton definitely lost his cool there, but the refs, for the second game in a row, were incapable of calling anything on the team not named the Sharks.