Semi-sextiles slowly bring back life in relationships, after disputes for example. Semi-sextiles are time on your side. It is the opposite of Murphy's Law, if things can go right, they are likely to go right if nothing disturbs the process. Semi-sextiles take care of the details for you.
Contrary to sextiles, semi-sextiles operate automatically. In fact, you need to explicitly opt-out of what semi-sextiles perform for you if you don't want it to happen (whatever the reason). Besides, whereas you are clearly aware of your options with sextiles, what semi-sextiles achieve can easy escape your attention, but not to the extent of operating unconsciously.
In synastry, semi-sextiles are a strong indication that chemistry will operate between involved energies. It is a big plus for the success of lasting relationships. Semi-sextiles also rule serendipity.
Related aspects: trine, grand trine and sextile
Other minor & rare aspects: grand-cross, quincunx & inconjunct, quintile & biquintile, semi-square, sesquisquare, t-square and yod