On a synth with patch memory and front panel controls, when a patch is recalled, the panel knobs, sliders, and switches do not indicate the current patch settings (unless the synth has motortized controls, which very few do). Most such synths are arranged so that moving a control causes the parameter value in the current patch to immediately change to the indicated value, and thereafter track the control, until another patch is selected from memory. However, when building a new patch, the performer often wishes to have all of the parameters tracking the panel controls, either because it is confusing not to, or because the controls are already mostly set up to the desired values. Therefore, the synth provides a button usually labeled "manual" which immediately transfers the current panel control settings to the current patch. On some synths, when manual is selected, the current patch name/number display is also erased, as a way of indicating that the current patch parameters no longer have any relation to the last recalled patch.
In the terminology used by organists, a "manual" is a single row of keys on a keyboard. (Organists consider both keys played by the hands, and pedals played by the feet, to be "keyboards"; a "manual" is specifically a keyboard played by the hands. This derives from the Latin and Romance languages which use some variant of "main" to mean "hand".) Most Hammond organs have two manuals, each of which plays a different patch, and pipe organ consoles often have three or more manuals with various complicated interconnections. Few synthesizers have been made with two or more manuals. Sequential Circuits made the Prophet-10 synth with two manuals, each capable of playing a different patch, and with various schemes for voice allocation between the two manuals. In the late '70s and early '80s, Yamaha offered several large console organ/synth combo units, such as the GX-1, with two or three manuals.