The "sustained chord" feature you describe is a function of the auto-accompaniment mode of "arranger" (one man band) keyboards and only works when auto-accompaniment is turned on. It only applies to the tones of the currently selected rhythm. It does not apply to a single tone that is assigned to the lower portion of a split keyboard. That lower single tone operates the same as the "upper" tone(s) - when you press the keys, it plays, when you release the keys, it stops. Now, when auto-accompaniment is turned on, but the rhythm is not running (that is: you do not press the Sync Start button or the Start button), the keyboard will produce a steady "drone" chord when you play a chord in the lower portion, but you have no control over the choice of tones in this chord, as those are assigned by the currently selected rhythm. It is possible to use this "drone" chord as a sort-of manual, if not somewhat boring, left hand accompaniment. The auto-accompaniment feature and the ability to assign a single tone to the lower section of the keyboard are typically viewed as two entirely separate performance features and not normally used together, as this can result in a very muddy output from the lower portion of the keyboard. That is to say, you would normally only play a rhythm OR a single tone from the lower portion of the keyboard, but not both together. A rhythm cord will be sustained, while a single tone will not. After all, if you have a piano tone assigned to the upper portion of the keyboard and an acoustic bass assigned to the lower portion, you would not want that bass tone to be automatically sustained. When a lower tone IS used with a rhythm pattern, it is normally done with the keyboard fingering in "Full Keyboard" or "AI" mode so that the auto-accompaniment interprets the correct rhythm chord from the individual notes being played across the entire keyboard, rather than you having to make a complete "chord" in the lower portion, thereby avoiding the aforementioned "muddiness". Most high-end boards allow assigning a sustain pedal to either the upper or lower or both sections of a split keyboard, but the lower priced models usually only allow pedal assignments to the upper portion.
