Bouncing Down (also called reduction mixing, "bouncing tracks", or simply "bouncing", ) is a multitrack audio recording technique which takes advantage of magnetic tape’s rewritable nature in order to circumvent its limited number of tracks. Even as analog tape recorders have grown in sophistication and track count, the technique remains an important tool in analog music production.
The technique involves combining multiple previously recorded audio signals. A series of discrete recordings can be printed (bounced down) from their individual tracks on the tape's surface to either an unused monophonic track, or panned across an unsused stereo pair of tracks, thus freeing up additional space for further overdubs. The process can be repeated indefinitely, resulting in increasingly layered recordings. However, additional machine noise, tape hiss, and tape saturation will be added to the recording with each consecutive bounce.
For example, if you have eight available tracks and you want to record a kick drum, snare drum, hi-hat, rack tom, floor tom, and cymbals, you could record these elements to six individual tracks, process each recording to your liking, create your desired mix, and then bounce them to the remaining stereo pair. You would then have all of your drum recordings in a stereo field on two tracks with six tracks now available to record additional instrumentation.

In the context of digital recording technologies, which since their introduction in the late 1960s have developed to allow for a near infinite number of individual tracks, bouncing becomes an esthetic or organizational decision rather than a necessity and the term often refers to creating a mono or stereo audio file from the digital multitrack in the Digital audio workstation.