Overlaying charts in synastry shows you where someone’s planets fall in your houses and where your planets fall in their houses. Many people know where these house overlays are but they don’t understand what they mean.
Is it good?
Your planets bring their particular energy to the house they occupy in the other person’s chart. If your Saturn falls in someone’s 3rd house, you bring your Saturn energy to that person’s 3rd house area of life. If someone’s Venus falls in your 6th house, then that person brings their Venus energy to your 6th house area of life.
These planet/house overlays are neither good nor bad. Whether or not you have good synastry with someone depends on what you want. What kind of connections will make you happy? Does having an easy everyday life, where you never argue about who is going to take out the trash, sound like heaven to you? Then maybe a 6th house Venus overlay is exactly what you are looking for.
It’s like a permanent transit
Planet and house overlays in synastry are like permanent transits. If you’re familiar with Saturn transiting your 3rd house, then you already have an idea of what your natal Saturn falling in someone’s 3rd house will feel like to that person. In a relationship that “transit” is permanent for however long you are together.
An example: What would a 3rd house Saturn overlay be like?
Saturn is the planet of restriction and hard reality. Saturn also shows mastery and teaching, and learning how to thrive despite limitations. The 3rd house is about communication, learning, teaching, siblings and quick daily movements. The double emphasis of learning and teaching, with Saturn and the 3rd house, immediately brings to mind a student/teacher relationship. Perhaps many of the “lessons” of this relationship will revolve around communication. Even in a romantic relationship this overlay can show that one person is learning from the other and that communication is something these two people will take seriously.
With your Saturn in someone’s 3rd house: you make them feel that they can’t communicate openly, you make them feel insecure about their intellect, you limit their daily mobility, you encourage them to take their education seriously, you teach them a new language, you restrict relationships with their siblings, you encourage commitment to relationships with siblings, you help them learn how to gather their scattered energy and concentrate, you feel that you have something to teach them about proper communication (“proper” according to whichever sign Saturn and the 3rd are placed), you feel responsible for their education, you want to have serious discussions with them, you make them stammer or stutter, they don’t know what to say around you…these are a few possibilities.