The natural horn, long used for hunting signals, found a distinctive role in orchestral and band music in the 18th century. A natural brass instrument is limited to specific notes and to enable the instrument to play in a variety of tonalities, changeable crooks of different lengths were supplied. The crooks are extra lengths of tubing that lower the pitch of the horn. An orchestral hand-horn with its many crooks can be seen in the photo below.
Players of the hand-horn placed a hand into the bell. By inserting their hand the player could play a wider selection of notes than was possible on the natural horn. This formed the basis of the hand-horn tradition, used during a period when the horn became popular as a solo instrument and entered the orchestra.
A modern horn uses valves, which changes the length of the instrument when different valves are pressed. Modern horn players also put their hands in the bell, but today this is done to tune the instrument and for making different sound effects.
Can hear the difference in tone when the hand position is changed in the video?