Some mediums go into a full trance, thus allowing the spirit to take full control over their body, allowing the spirit to speak through the medium directly. Other mediums use a meditative state to connect to spirit. This can be done in a light trance or disassociated state, in which the channeler is partially or totally aware of the process. The channeler's voice may or may not change.
Other methods for channeling consist of automatic writing, in which the hand appears to move by itself to scribble words. Others use a form of telepathy, receiving thoughts from spirits in their minds, by sensing which thoughts are theirs and which are spirits, they learn to distinguish and can pass on the information received with just as much clarity.
Often, not only thoughts can be transmitted, but also images and even feelings. Some mediums appear to hear voices which others cannot hear. It is not clear whether this is a physical phenomenon or a form of telepathy.
Everything is frequency. When one channels they are tuning into a particular station or wavelength within the universe. Much like tuning into a particular channel on the radio. You just turn the dial through the various stations and pick a station. Each station brings a specific frequency of information that we sometimes associate with colors, chakras and tones.
Channeling has always been part of human connection to higher frequency of thought, as we are experiencing multi-dimensionally. In the most liberal interpretation channeling can include the processes of imagination, intuition, inspiration, and premonition.
We all channel. We hear thoughts in our minds, our dreams, and receive messages in one form or another from other dimensions within the grid. Being aware of these thoughts is what creates consciousness. Becoming consciousness will often increase the speed of awakening to self. Channeling is just another tool in the process.
Channeling dates back to early human history, as shamans have been using some form of channeling over millennia, and became popular in the mid 1800s. Some of the best known mediums are Edgar Cayce (1877-1945), Aleister Crowley (1875-1947), Jane Roberts (1929-1984) and Allan Kardec (1804-1869), who fathered the term 'spiritism'.