Re: Suwari waza
I have done and do lots of this. When talking principles, well it is useful for practicing many different ones so there is not set hard and fast must be this or that.
The aim or you could call it game is for one person to grab (usually the wrists) of the other and stop them being able to do anything in response to their hold or pin. The other fella, being in seiza is thus unable to move as he would if standing. His job is to learn how to harmonize and respond in such a way as to manoeuver the other person no matter how he tries to stop you.
In Ki Aikido it can be used as a way of developing center and thus learning to move from center and develop power from center for you can't rely on legs. Physically it also teaches how, now that your legs are 'missing' how useless it is to just rely on your shoulders and brute strength and so you learn how to develop hip movement. Subtle hip manoeuvering to generate power.
Many things can be practiced from this position for it is a position where you can't run away for example and yet you have to learn how to be effective from it.
You can also learn to use the paths of energy redirection from it, circles etc.
This can lead on to a practice where another is allowed to attack you, be it to grab, to hit or even to kick from a standing position whilst you must remain on your knees and handle it. Different from suwari waza but I think you'll see the progression.
Put very, very, simplistically and physically you could call it wrestling from the knees or even judo from the knees if you like.