illustration from Barbara Brennan, Hands of Light, Bantom Books 1987

The tertiary or minor chakras are not part of the main circuit of ch'i (the microcosmic orbit).  Rather they would seem to correspond to points that regulate energy flow.  They are variously and unsystematically described in some New Age material and in the and in holistic health movement, radionics, and so on.  Although the above diagram from Barbara Brennan's book (which in turn seems to be based on David Tansley's material, assumes "twenty one minor chakras", this is a purely arbitrary number (an attempt at multiples of seven perhaps), and most certainly incorrect.  Other books give different numbers of minor chakras.

Some minor chakras, like the chakras in the palms of the hand, are used in healing work.  In my experience of practice with Reiki it seems that these constitute a definite sensation.  There are also definite minor chakras, not shown here perhaps because of prudishness on the part of the original author, on or in the genitals, as well as the eyes, nipples, and other sensitive parts of the body.