That’s Rosa’s left hand. The other was a right hand.
1) While puentes is sitting, the left hand grabs the seat.
2) While puentes is standing the right hand extends into the field of view.
The sequence in the video is 1) then 2); I said so in my post.
Also, during both 1) and 2), Rosa is using both hands to open her vest.
And puentes is using both hands to hold the camera and/or open his vest; right arm is not grabbing seat back.
And Mr Key is already past both of them on his way to the door.