Role projection
In romantic or sexual relationships, role projection or role transference is a technique that is used to allow partners who otherwise have incompatible gender or sexual roles to be involved with each other. For instance, it is used to allow an exclusive bottom to be involved with an exclusive bottom, or an exclusive top with an exclusive top. The technique combines role reversal with psychogical projection.
Projection and transference can be difficult concepts to understand. Basically, they're situations where someone takes emotions that originate in one person, and they imagine that those feelings instead originate in someone else. Usually projection/transference is done unintionally and subconsciously, but in this case, it is induced intentionally in order to achieve a specific goal.
Examples
Role projection can be used in many aspects of a relationship, from flirting to intercourse. For instance, if a bottom is interested in flirting with another bottom, one person has to hit on the other. However, this doesn't give any satisfaction to the person who is acting like a top, because they really only get enjoyment from bottom experiences. Usually, there would be little chance for chemistry in this situation. To make a romantic connection possible, one bottom imagines themselves in the other person's body. This allows them to enjoy the experience of being hit on, even though they're the one who is going through the motions of hitting on the other person
The situation is similar for intercourse. A bottom enjoys being penetrated, but because they're playing the part of a top, they have to go through the motions of doing the penetrating. To get enjoyment from the experience, they imagine themselves in the other person's body, and imagine what the other person feels like being penetrated. This doesn't mean that their actions as a top have to be subdued or fake; the more convincingly they play the part, the more the bottom (both they and their partner) will enjoy the experience.
Occurrence
There are a number of marriages where an apparent cis-man gets married to a heterosexual woman, but later realizes that he is is a trans-woman. In order to save the marriage (for example, if they have children at this point), he continues going through the motions, and plays a tradtionally masculine role. But in order to get enjoyment from the relationship, he needs to express his femininity, and so he imagines himself in her role during romantic and sexual situations. There are reports that role projection is one of the few things that allows these marriages to continue.
Another place this occurs is heterosexual men who are bottoms. There are many more male bottoms than there are female tops. This fundamental supply/demand imbalance means that only the most desirable men will have their needs met. For the other men, they can only form relationships where they're forced to play the part of the top much of the time (at best, maybe they can find a partial switch) Role projection allows them to still get their needs met somewhat.
Honesty and openness
It may be possible for one partner to play their part so thoroughly and consistently that the other partner never realizes that role projection is occurring. For shorter-term relationships, it may actually be desirable to conceal this fact. However, for longer-term relationships, the extensive deception is a hindrance to intimacy. It may be important to disclose the fact that role projection is occuring. This doesn't mean that the behavior stops at that point, as it is fundamentally necessary for the relationship to exist. However, acknowledging that role projection is occurring my lead to greater intimacy than would otherwise be possible.