How does tap tempo work? I don't understand the concept of it - so how can I use this to my advantage?
The G-Major has a global tempo at all times (when switched on, of course), which will remain the same, until you change it. This tempo is shown in the display when you press the TAP button once. Then the display will show the current tempo in milliseconds - and after a while the display flips back to its normal state. Or, you can get an idea of the unit's current tempo just by checking out the flashing intervals of the TAP button's LED.
Hitting the TAP button more than once within a relatively short period of time, will change the G-Major's global tempo. The new tempo is instantly displayed in milliseconds, and it is set by the time interval between the last two keystrokes on the TAP button.
You can choose to have the G-Major's tempo shown as BPM - beats per minute - instead. This is done by pressing MIDI/UTIL, and scrolling to Tap Unit, and then setting it to "BPM".
The TAP tempo is a very useful feature, especially when playing live. Assigning f.i. the delay time of a preset to the TAP tempo will enable you to control the delay time on the fly, via a simple momentary foot switch (or a MIDI board of course). As you don't always play songs in the exact same tempo, and as you probably use the same presets in different songs, obviously the Tap Tempo feature is very convenient.
Any algorithm that has "Tempo" among its parameters, can be controlled by the Tap Tempo. When an algorithm's Tempo parameter is set to anything else than "Ignored", the Tap Tempo will control the relevant parameter of this specific algorithm. In case of the delay algorithm, of course the delay time parameter is assigned. The arrow next to the parameter value indicates that this parameter is being "remote"-controlled - in this case by the Tap Tempo.
How the parameter in question relates to this tempo is up to you. The tempo parameter of any algorithm can be set to different beat subdivisions, like 1, 1/2D(dotted), 1/2, 1/2T(triplet), 1/4D, 1/4, and so on...