Oddly enough it "may" have been Yamaha. At least my PSR8000 uses this "single-finger mode". I guess if you do not know what a minor third is, it might be useful when using a fake book. I personally cannot use this mode. Is this what the Technics does? Do the PSR9000 and 9000Pro foresake this?

The only other arranger (or quasi-arranger) I have is the Karma. It is unusual, I think, in that it "recognizes" a chord no matter what you play with one finger or twenty. Obviously, there is no "chord cancel" function as there is on the Yamahas.