HAND POSITIONS: In your initial Reiki class (Level I or First Degree) you should have been taught a specific set of hand positions referred to as "holds" in the Reiki world. I have found, over the years and having read over 70 or more Reiki books, that these hand positions will vary greatly depending on who your teacher is.
The initial set of "holds" are just your foundation positions so you have a grasp of where it is best to place your hands when doing a full body Reiki treatment on someone else or yourself, and they generally correspond to the 7 major chakras (root, sacral, solar plexus, heart, throat, third eye and crown) and all the major organs and major joints of the body, and the brain.
Once you've been doing Reiki for awhile however, you are encouraged to move beyond the foundational positions and go to where you feel guided or lead to go, or just have an intuitive knowing that your hands need to be here or there, in a special way, not necessarily the way you were taught.
Including the chakra system into Reiki treatments was not really a part of the original system beginning with Dr. Mikao Usui, the creator of the system. No one is really sure when or who added them in, but we could safely say sometime after Mrs. Takata's time, probably through one of the people she attuned to Reiki Master Teacher before she passed in the '90's.
SWEEPING THE AURA: You are also taught in Level I how to "sweep" the aura (biofield, energy field) of your client after a Reiki treatment. Again, there are a myriad of ways to do this, some even involving using crystal wands, feathers, singing bowls and even drumming. Typically though the hands are "raked" or "combed" through the energy field of the receiver (person you are giving Reiki to) from head to toe, and this is performed a minimum of 3 times. I like to think of it as removing the "dust bunnies."
It's done by dragging or raking your hands over the body with the intention of removing any dirty or negative energy from the aura that might have been released during the session, or that they might have come in with, and then you may do several different things with it. Some throw what they've removed into a candle flame, some into a Violet Flame they've mentally constructed, and others send it up to the light to be transformed to a more healing energy for whatever you'd like to send healing energy to.
You start by sweeping from above the head (remember energy field starts several feet above the top of the head) to several feet past their physical feet also. Additionally make sure you are taking your hands way out to their sides, as the aura extends several feet out on the sides of the body as well, not just above and below it.
In another energy medicine modality I'm trained in, we do this BEFORE we start a healing session. I like the idea, kind of starting with a clean slate!