Well, this is also seems to be a lock-up in S mode at ~115km/h.At 53 mph the transmission goes into lock -up if the conditions are correct. In this condition, there is theoretically no flow of oil from hydraulic pump to transmission motor. Efficiency increases 10- 13%. This reduces fuel consumption at cruising speed similarly.
The jerk you feel is the transmission going into or out of lock-up.
At 70 mph the transmission will be in lock-up unless you upset this condition in manual mode.
Regards Lenny