Different couples may choose to dance in ways that place different requirements on the amount of physical connection pressure between the man's wrist and the lady's back. For sublimely skilled dancers who each move their body in a way compatible with their partner's needs, very little pressure is required for perfect communication. On the other hand, many developing couples rely on a quite forceful frame connection to help lock-in the relative body positions despite various mistakes in leg and foot usage that would ordinarily force the bodies out of position. When the frame is given a physical role, his job is to hold the couple together with an inward force, while her job is to push outwards to keep the couple from collapsing.
If a couple decides that a physically meaningful frame connection will be part of their method of dancing, then we might ask where the force that lady pushes back against the man's hand with is to come from. If both dancers are expert in leg and foot usage, the lady could exert a carefully measured force against the man's wrist by pushing against the floor with her legs and using a tiny fraction of her weight, but few who rely on a physically forceful frame have yet developed the skill and precision do to so consistently. Most will require a way to directly push back against some part of his body. One practical way to do that is for the lady to place her left wrist more to the inside of the man's arm, and thus be able to push against his bicep with her wrist and thumb in order to balance the pulling force he places on her back.
But the most commonly seen approach amongst those competitive dancers who wish to have a rigid frame is to place the lower rib cages in forceful contact such that pressure there pushes the bodies apart to balance pull from the man's wrist holding them together. While this achieves its intended goal, members of the public may find it a bit too intimate for social dancing. More importantly, dancers who try to achieve a fixed body connection before they have developed expert skills for standing leg usage tend to create the connection by arching their backs and pulling their hips back to create room for the legs, rather than by projecting their weight towards their partner while maintaining an aligned lower spine.
For those who do not wish to emulate the look of high level competition dancing, the simplest approach is probably to have a moderately firm frame connection in the arms, while keeping some space between the bodies both as a social buffer, but also to loosen the degree of perfection required in standing leg usage. This isn't a bad approach to take while building basic movement skills in beginner competitions, either, since it helps each dancer learn to carry their body on their own feet.
