Well it's not an 'aspect' at all, it's an aspect pattern. That is, it is a figure made up of the lines drawn to join planets in aspect.
Aspect patterns are a modern conception, you won't find any mention of them before the second half of the last century but many modern Astrologers except the Yod as important, though it seems few make a great deal of use of it.
The yod is two planets in opposition connected to two other planets, the first of the two in opposition is connected to the other two by semi-sextiles, the second planet is connected to the other two by quincunxes.
Now to be honest, I just keep to the classical or major aspects. If there are quincunxes, I MIGHT pay attention if I think they would add something. I don't use semi-sextiles or semi-squares under ANY circumstances. They are just too weak to convey meaning that hasn't been conveyed by other chart factor.
Arguably the two 'additional' planets provide alternative ways of discharging or expressing the tension of the opposition.
I don't even pay much attention INITIALLY to Grand Trines and Grand Crosses. ONLY if I think they add something special, that the aspects alone have not conveyed and is not conveyed elsewhere do I include them. But again, to be honest, I have found very few if any circumstances in which I've needed them. That is not to say that other Astrologers might find them useful, it depends on your approach.
However, Astrology, like many things' goes in fashions, and there is a fashion for making a lot out of aspect patterns. Therefore if you're an Astrological Counsellor, Yod's may be an important part of your amoury for helping people deal with their issues.
