Check out this very helpful video from the JJAU on the technicalities of the mount position.
"When in the top position, control is only awarded if the knees are below the shoulder line. This is when the guard is passed and there is no half-guard. It is established when the athlete is sitting on the torso of the opponent with both knees or one foot and one knee on the mat, positioned facing the head and with u
p to one arm trapped under the leg. Control of the torso is only considered if the knees are under the shoulder line. If there is an uninterrupted change from mount to back mount or back mount to mount, the MR will award four points for each mount position given that each position was secured for three seconds."