The Munter, or Italian hitch is a knot used often by climbers and cavers. It depends on friction and the belayer's or abseiler's hand to keep the weight from sliding more rope through the knot. This knot supplies a smoother ride than a belay plate such as a GriGri or an ATC (Air Traffic Controller).
It is pictured below as it would operate on a belayers harness. The loop represented by a whole buncha Cs is the carabiner and is attached to the belayer's harness. If the belayer were on the top of the cliff instead of the bottom, the belayer's harness, in turn, would be attached to a piece of protection, a bolt, or something else that won't allow the belayer to be pulled from the top of the cliff by the climber's weight.
It is important to note that, as it is pictured here, the knot will pull through to the climber's side of the carabiner. This will not change the effectiveness of the knot.
________________
/ \
/ ___________ \
/ / \ \
/ / CCC \ |
\ \ CC CC | |
\ \__CC___CC___| |_____
\ CC | | \
\____CC________| |___ \
--To climber CC CC | | \ \
________________CC___CC___|___|___/ /
CC /
________________CC__________________/
CC CC | |
CC CC | | To belayer's hand
CCC | | |
V
IMPORTANT: Don't try these things without the proper equipment plus expertise, or someone else who knows what they're doing. Instructions are not a replacement for experience.