How does the whole Korra/Wan and Raava union work? What actually happens to Raava's spirit once she rejoined with Korra? Does Raava's essence lay dormant, deep within Avatar's soul until needed? Can Raava control Korra if she wanted to? Does Raava see and hear everything Korra sees and hears? Most importantly, why dosen't she communicate with Korra more often, or with any other present Avatar? It was proven that they can talk to each other, when she motivated Korra, just before Unavaatu was about to crush Korra to death. It's like when she fused with Wan, after they defeated Vaatu and Wan closed the spirt Portals, Raava went silent. Even at the end, before Wan took his last breath, it was unclear whether or not Wan actually heard Raava's statement about her being with him forever.
As I understood this:
Raava fused with the spirit of the original Avatar (Wan). Their souls merged, to became one, so the human soul of Wan is now immortal and immutable, but his human body is not. So, when he dies, the soul migrates to a new body, who has to "awaken" to his true identity, hence, the "Avatar State", where Aang, Korra, or whichever Avatar, stops being him/herself to become simultaneously ALL the avatar incarnations. Raava then would not only be a fundamental part of the Avatar soul, but a link amongst the incarnations. That's why being killed in Avatar State is the only way to stop the cycle of reincarnations, because one would be killing Raava and the human host at the same time.
Anyway, this is my interpretation, don't know if there is a canonical answer, but I don't think it would be substantially different.
Ravva stayed inside the Avatar and every time in the Avatar state she would let out her power this is proven cause Roku rip said that:
Avatar Roku: The Avatar State is a defense mechanism, designed to empower you with the skill and knowledge of all the first Avatars. The glow is the combination of all your past lives focusing their energy through your body. In the Avatar State, you're at your most powerful... but you are also at your most vulnerable.
In TLOK we see that Unavattu take out Ravva from Korra and destroy the avatar so she lost the connection to her past lives but she could still go in the avatar state so That's because Ravva was giving all of her power to the avatar state