This ter'angreal is what is used to test a woman to become a full Aes Sedai. A number of channelers must channel all Five Powers in a complex weave into it, while the candidate enters the rings into a mirror world and must successfully perform one hundred weaves at the right times to pass.
In the test for Moiraine the Oval was activated by seven Aes Sedai, one from each Ajah. In Nynaeve's test there were seven Sitters and the Amyrlin Seat. In both cases the Sister involved were strong in Saidar, but it is not stated if a minimum amount of power is required to participate in the activation.
Unlike the use of the Silver arches ter'angreal, the sisters activating the Oval ring can see what is happening within it and can actively put particular tasks on a tested person. It may result in unequal difficulty of particular tests, depending on personal relationships between a tested and a tester.
Injury and death can occur within the rings, and a person can also simply not emerge from the ter'angreal.
The ter'angreal probably uses Tel'aran'rhiod for creating virtual reality. Therefore, thanks to her experience, Nynaeve was able to break some rules of the test, for example, to channel where she shouldn't be able to.
Moral controversy of the test
The test itself, as practiced in the White Tower, seems to be testing the ability of a tested person to keep the rules (e.g. not use balefire) regardless the suffering of other people displayed in the test, the ability to leave them in a danger and continue (a tested person don't know they are not real). Nynaeve thinks that this way Aes Sedai detach themselves from the world and become arrogant.
Quite surprisingly, some tasks in the Silver arches ter'angreal are of the same kind, though they are created by the ter'angreal itself, not by Aes Sedai.==Notes==