Time for a bit of a workout, with this one.
The Giver (the one doing the sucking) is actually in a prone position, usually on a bed, where there is an edge/side.
They lay prone, with their head hanging off the edge of the flat surface (ideally a bed).
One might want to have their legs bent upwards, for added comfort.
Make sure that when dangling one's head off the edge, that it is comfortable, and no sharp or hard edges are pushing into the back of the neck.
The Receiver (the one being sucked off) generally will need to either squat, or bend down to enable proper penetration of the Giver's mouth.
The closer the Receiver stands, the more he may need to bend over, or squat.
Experiment with different positions, of squatting or bending, to afford a better angle for insertion, and for continued in and out thrusts.
However, being closer allows him the opportunity to caress and fondle the Giver, adding to their pleasure.
Care needs to be taken, in the strength of any thrusting motions, by the Receiver.
Practice (Associated Acts):
It is possible, that if the Receiver turns around, they could also get a rather nice rim job, but they may have to grab hold of their butt cheeks, and spread them for better access.
Depending on the Stature of the Receiver, the height of the furniture being used, will determine the actual position the Receiver will have to maintain, for a good session of sucking.
It should also be noted, that care should be taken, to insure the Giver is able to breathe, while having their mouth stuffed with the penis.
Care in the angle of penetration should be taken. An extreme angle can cause damage, or discomfort for all parties involved.