In Yesod we find the equilibrium of the Astral Plane, the Sphere in which both Netzach and Hod find their balance and perfection. At first sight Yesod seems to be a contradictory Sephirah. At one side it is all fluidity, ruled by the Moon in a constant state of waxing and waning, of flux and reflux. On the other hand it is called 'the machinery' of the universe, being the receptable of all emanations of the other Sephirah above it on the Tree of Life, and the only transmitter of these emanations to Malkuth.
When we think of it it is not so contradictory as it appears. A machine has to move, at least something in it has to, in order to function. With no movement and rhythm whatsoever there is nothing the machine could empower. Unlike Netzach Yesod is no uncontrolled mess of instincts, it is the Sphere of reflection and imagination emanating from Hod, the wisdom and the structure. Yesod can be compared with a giant image gallery storing the images of past and present, of below and beyond, beautiful ones and ugly ones, soothing and frightening, dreams and nightmares. They're floating and shifting, moving like figures on an old silent movie screen.
And that's what they are considering that Yesod belongs to the Astral Triangle, the realm of illusion. With the Moon being seen as the ruler of the tides and thus Water Yesod is related to the Seas of Binah and in fact it can be also compared with a deep sea. And like the Moon who has no light of its own the light we see in Yesod is but a reflection of Tiphareth which lies above it on the Pillar of Mildness. Yesod is called the 'Vehicle of Life' because it is the vehicle that channels all life into Malkuth.
Given that Yesod receives all the emanations from above while securely resting in the Pillar of Mildness, also called the Pillar of Harmony, it is a very powerful Sephirah which in turn renders the Tarot cards assigned to it into powerful cards.
See below the Tarot cards corresponding to Yesod - the Nines: