by Rev. Jason Storm
Symbols connect your channel to higher levels of Energy
When you take your attunement to Level II, the above three symbols will be permanently installed in your HEC. They will be drawn into your hands so that you can access their power in your treatments. These symbols are like little amplifiers that draw more Reiki to you and allow you to dramatically increase the levels of energy and the types of energy you channel into your treatments.
Some more recent sources and possibly Dr. Hayashi as well, have said that if you are not attuned to these symbols you cannot invoke their power or work with them in any way. But, as countless others have proven, this is not entirely true. You can tap the power of these symbols without having been formally attuned to them, but if you have had an attunement to them you connection will be stronger (read that again: your connection will be [only] stronger -- not necessarily better).
I fully believe that if you ARE NEVER attuned to these three symbols, but use them anyway, over time and with practice your connection to them will grow at least as strong as someone who has been attuned to them. Energy flows where the mind goes! I think of it in this way. The symbols are like switches that turn on an electric light in your bathroom. Before you can turn it on it has to be installed and connected to the power in your house. If you are ignorant of the symbols, then they are not installed. If you at least know what they are, they are at least partially installed. If you are attuned to them (as you will soon be) then you can reliably access their energies any time you like.
The traditional method of invocation is to draw the symbol over the area to be treated (either physically or mentally), then say it's name three times and flow Reiki through it. Many people can simply visualize the symbol in its entirety, say its name, and "send it whole" (and this is my preferred method. I suggest you make it your goal to use the symbols in that way as well).

Cho Ku Rei
"Let all the power of the Universe come to this point NOW!" or more simply, "Put the power HERE!" is the meaning of the first symbol. It is commonly referred to as "the #1" or "CKR" or simply the "power" or "focus" symbol. This symbol is drawn from the "handle", down, then spiralling three times inward. Traditional sources suggest that the clockwise CKR is for taking energy out, and the counterclockwise is for putting energy in. Experiment and see what you feel and use that which works best for you.
After you take your attunement and have this permanently installed, you will be using this symbol with every treatment you do. You can place this on the entire body of the client at once to amplify everything that you do with them treatmentwise. It can be used to increase the positive "vibrations" and healing energies in a place that you wish to do treatments by simply placing it over the area in which you want increased energy. It can be put into foods and drinks to increase their positive, nutritive qualities, or to make "Reiki Elixirs". Its energy both amplifies and seals, with your intent being the governing factor. If you invoke its use for amplification, then amplify it shall. If you place this over the client at the end of your treatment with the intent that it seal the healing energies in, then so it shall be done. This is the most common symbol in all systems and styles of Reiki.

Sei Hei Ki
"Man and Buddha become one" or "the Mental Symbol" are the traditional meanings of Sei Hei Ki. It is also called the "#2" or "SHK". Its purpose is to connect your mind with the infinite mental consciousness of all beings that have ever existed or will ever exist. It can be used for clearing or helping resolve mental issues or problems (such as depression, anxiety, worry) and can be used to "implant" positive affirmations the client has asked for. As a tool for changing or eliminating habits, SHK is very effective. Simply place this symbol into the mind, say its name, and give the affirmation or suggest the change of habit the client is requesting. Remember to make only the changes the client has requested, as their mental body will reject anything that this not a part of their own need for healing.
In doing a "mental" treatment, use the name that client is most commonly called by. This is to attract the subconscious/unconscious mind of the client to the time and place of the treatment. Let the person or the symbol itself show you the way to proceed in giving treatment and where to treat and when. It is very common for you to have an image or feeling that you need to be using a particular symbol at a particular time. Always listen to this feeling as it is the power of SHK guiding you. SHK can also be used to clear negative energies from a person or place, and can be placed over windows and doors to clear them of negative energies and bind them from allowing negativity to enter. Place SHK's over medicines and foods to increase the healing properties of them.

Hon Sha Zei Sho Nen
"The Buddha in me bestows upon the Buddha in you the Truth of All Truths" is the traditional meaning of the "#3" or "HSZSN" symbol. It is also known as the "distant healing" symbol as it is the symbol that builds the bridge from consciousness to consciousness. When Usui was teaching his first students how to connect to the energies of Reiki, he used this symbol to convey to them a means by which they could consciously connect to the minds and bodies of others without them being physically present. This symbol also works predominantly on the emotional level and in the emotional body.
The following is from my Reiki Master (reprinted with permission -- Thanks, C.J.!:-) and describes pretty well the traditional thinking regarding the #3 symbol.
"On a higher consciousness it connects person to person (with no ego to get in the way). In this dimension there is no time or space. Things are not either/or, there are many levels of truth, truths change with growth or spirituality for the different levels. HSZSN transcends time and space with the use of this symbol. HSZSN works on the person's subtle body and filters from there into the physical body. A dense body is one that participates in the eating of meat, smoking, drinking, taking of drugs, etc. Treatment from the higher levels of consciousness are more effective, take less time phsically, and bring about better results (you treat the cause instead of the symptom). Make appointments with people to send treatments. Tell them to sit down and do nothing (this is to let them know that you are taking time and energy to send THEM energy). People feel better after treatments but unless they really understand what Reiki is about, they will just go back to their lives and not think about their aches and pains anymore and certainly not give a thought to you and Reiki having anything to do with it!
"Three or four treatments are generally required. If there are serious problems, treat everyday for as long as necessary. Keep in mind, they can have the same physical reactions as someone receiving hands-on treatments. The physical symptoms may get worse before they get better, they may crave water, they may not sleep well for while. All the [detoxification] symptoms you get with hands-on happens just as readily with distance treatments."
--Rev. C.J. Aberte, RM