I didn't watch the video and cannot properly explain it, but there is direct mechanical connection from ICE to wheels. AFAIK both MG1 and MG2 can work as motors and generators and spin in either direction. If you apply proper counter-torque by one of the electric motors (for planetary ring), the torque effectively flows from ICE to wheels. I'm Prius driver myself, it's a nice car for us who cannot yet drive full electric vehicle for a reason or another.