The tent is air tight preventing the loss of moisture into the environment. Contact systems are where thin plastic of say 80 - 120 microns is laid directly on top of the cuttings. The freshly watered cuttings will release moisture into the small air gap resulting in a high humidity. The condensation of the humidity on the plastic will form a clouded effect which will act as a shade barrier thus reducing light levels into the plastic keeping it cooler. The environment will also remain cool due to the continual cycle of water evaporation and condensation.
Contact systems are better in summer and tent systems better in winter.
In summer leaf turgor is of primary concern so the contact system is very good at keeping the leaf moisture at high levels. At higher temp there was less water stress under the contact systems due to a lower leaf temperature. The simplest enclosure is a plastic bag placed over a pot within an internal framework provided by bent coat hangers. The bag is folder under the pot to give a tight seal. The unit should be placed in a fairly light place but never in direct sunlight, which will cause overheating within the bag.