This section is from the book "A Dictionary Of Modern Gardening", by George William Johnson, David Landreth. Also available from Amazon: The Winter Harvest Handbook: Year Round Vegetable Production Using Deep Organic Techniques and Unheated Greenhouses.
"A layer is a branch bent into the earth, and half cut through at the bend, the free portion of the wound being called ' a tongue.' It is, in fact, a cutting only partially separated from its parent. The object of the gardener is to induce the layer to emit roots into the earth at the tongue. With this view he twists the shoot half round, so as to injure the wood-vessels; he heads it back, so that only a bud or two appears above ground, and when much nicety is requisite, he places a handful of silver sand round the tongued part; then pressing the earth down with his foot, so as to secure the layer, he leaves it without further care. The intention of both tongueing and twisting is to prevent the return of sap from the layer into the main stem, while a small quantity is allowed to rise out of the latter into the former; the effect of this being to compel the returning sap to organize itself externally as roots, instead of passing downwards below the bark.as wood. The bending back is to assist in this object by preventing the expenditure of sap in the formation or rather completion of leaves, and the silver sand is to secure the drainage so necessary to cuttings.
"In most cases, this is sufficient; but it must be obvious, that the exact manner in which the layering is effected is unimportant, and that it may be varied according to circumstances. Thus, Mr. James Munro describes a successful method of layering brittle-branched plants by simply slitting the shoot at the bend, and inserting a stone at that place; (Gardener's Magazine, ix. 302;) and Mr. Knight found that, in cases of difficult rooting, the process is facilitated by ringing the shoot just below the tongue about midsummer when the leaves upon the layers had acquired their full growth; (Hort. Trans, i. 256;) by which means he prevented the passage of the returning sap further downwards than the point intended for the emission of roots. It will sometimes happen that a branch of a plant cannot be conveniently bent downwards into the earth; in such cases, the earth may be elevated to the branch by various contrivances, as is commonly done by the Chinese. When this is done, no other care is necessary than that required for layers, except to keep the earth surrounding the branch steadily moist." See Fig. 97. L E A D W O R T. Plumbago.