Semi-squares suggest a refusal of integration or unity. The astral items involved in this aspect are like magnets repelling each other when they get too close. They are not to be confused with squares which demultiply energies, or t-square which suggest strong personality traits.
The semi-square energy is similar to a refusal to connect or to come to a conclusion. It can be a refusal to go with the flow. It may lead to eternal procrastination and can irritate others. On the positive side, they contribute to neverending story, which can be a good thing in synastry.
Semi-squares are neutralized by conjunctions or perseverance. They aggravate quincunx and opposition energies but facilitate the liberation of squares and sesquiquadrate energies. They get trines energy into motion, like light through a prism creates a rainbow.
Related aspects: square, sesquisquare, t-square and grand cross
Other minor and rare aspects: grand trine, quincunx & inconjunct, quintile & biquintile, semi-sextile and yod