When she raises her thighs and keeps them wide apart and engages in congress, it is called the "yawning position".
HER
Lie back and let him control the motion
Squeeze his chest between your knees for increased friction, or spread them for deeper penetration
Try riding close to the backrest and letting your man slide up and down, the intense clitoral stimulation coupled with the full body contact is absolutely unforgettable
HIM
With her positioned towards the front of the chair, motion is created by grasping the side rails and pulling yourself in and out
For something she'll never forget, position her close to the backrest, lay on her and grasp the back of the chair, plant your toes on the front bar and push
Using different muscle groups, pulling with your arms and pushing with your legs, is a good "rest", plus the full body contact can be extra arousing