Also note that while I recognize that (most of) these positions can be done by two men or two women, for ease of use, I chose to use the terms that would apply to the writer, interspersed with generic terms like "performer" and "receiver."
The CartwheelIn this acrobatic position, the receiver stands on his head, against a wall, using his hands to steady him (like he’s about to do a cartwheel). The performer stands in front of him with her back to him and her feet at either side of his head. She then bends over and allows her partner to insert his penis. Yes, really.
Challenge: In this position, the male partner has to remain balanced, breathing and relaxed enough to stay hard AND not faint from the blood rushing to his head. His partner doesn't have it easy either: She has to set the momentum while keeping him at an angle that’s comfortable for both while not tipping him over.
Pair of TongsIn this position, the woman lays on the bed sideways with half her torso hanging over the side of the bed, supporting herself with one arm on the floor. (For an added challenge, do this freestanding, as shown below.) The man, stands facing her and grabs hold of her waist. He then uses his leg to hold down her lower leg, while he holds her upper leg and inserts his penis.
Challenge: The woman is going to have to have fantastic upper body strength and balance to stay steady here. Her partner won't exactly be slacking here either, as he'll have to support much of her weight too. It might just be more exciting than a typical yoga class though.
The Big and Little DipperThese two require strength, but they aren't entirely unrealistic. In the Big Dipper position, the man sits on the couch, stretches his arms behind him and puts his feet on a coffee table or ottoman. The woman straddles him, while he uses his arms to push himself up and down, hence the name. Then there’s the Little Dipper, in which these positions are essentially reversed. The man lays supine on top of a big pillow on the floor next to the couch. The woman assumes the position of the male in the Big Dipper except she rises and falls onto her partner's erect penis.
Challenge: The person with the penis (or a strap-on!) has to be in impeccable shape to do what your personal trainer might call "triceps dips" for any length of time. In addition, the person doing the work has to have the concentration of a monk to remain balance and continue to hit the target. There are way too many opportunities to miss here and thrust a buttock down on an erect penis. And that, my friends, is how your evening turns into date night at the emergency room.
The Overpass (or the Torture Twister)The fact that this one has the word "torture" in its name should provide a hint as to its difficulty. Here, the woman (or receiver) lays on her back and lifts her feet up and over her head so her toes touch the floor behind her head. Her thighs are resting against her torso and her knees are by her ears. The man (or the performer) straddles himself over the woman’s genitals, putting one foot by the receiver's head and one by her side or back. The performer then thrusts, like he’s getting up and down off a chair, into the anus or vagina.
Challenge: The woman or receiver, must stay steady while her internal organs are being scrunched against her ribcage and her partner's thrusts put continued pressure on her shoulders and neck. But he has challenges too. He must angle his upward facing cock into a downward position for entry or bend over his partner so he can enter at a more convenient angle. This position is not for amateurs. If you are able to complete it without injury, I give you serious props. If you can manage to enjoy it, you're a true sex ninja.
The Booster SeatIn this position, both partners face the couch or bed with the woman (or receiver) in front of the man (or performer). While holding her waist, she puts her feet on the bed while he holds her up so that she appears to be sitting in a chair. The man can enter her vaginally or anally, and help support her as she moves up and down.
Challenge: The male (or the female with a strap-on) definitely needs incredible upper arm strength and the woman (or receiver) needs thighs of steel. Momentum is the key here, but if you are in-sync and in-shape you might be able to pull this one off.
The Anal Bumper CarsWhee ... this one sounds like fun. But I'm not sure it's possible - or a good idea. Here, the lovers position themselves stomach down, facing in opposite directions with the man (the performer) on top. The woman should shimmy down toward her partner's feet so that his penis is lined up with the anus, as she is face down under him, with legs spread apart. He then proceeds to enter her anally.
Challenge: I cannot imagine any situation where this might be physically possible. Even if there's a penis long enough for this out there, it would be at a downward angle, which, as we know, is the exact opposite natural direction for an erect penis. If you try this one, be very careful not to stretch the performer’s cock farther than it should comfortably go or you might end up at the emergency room (again) making up far-fetched stories. (Anal sex carries a few risks. Learn more in Why You Shouldn't Have Anal Sex.)