Video 2 - Class 14B Tech 84: Collar choke from the back starting from Side Mount.
This 2nd technique comes off of the same initial situation as the previous arm lock. In looking to create a scramble and escape, the opponent has momentarily exposed their back to Prof. Rodrigo.
Please pay attention to how when the opponent posts their rear hand on the mat to sit up, Prof. Rodrigo uses his leg to capture the arm and eliminate the ability to defend a collar choke attack.
At the higher levels of Jiu-Jitsu, we often see the back attacker use their legs to kill the movement of one of the opponent's arms to take away their ability to hand fight.