Horizontal aftertouch was a feature on the old Crumar Composer analog paraphonic synthesizer.

Perhaps they licensed this feature from Yamaha, I really have no idea, but it was surely a terrific (and very natural) way to add vibrato.

Ian

I should add that Yamaha's Horizontal Aftertouch, besides being used to create vibrato, could also be assigned to affect pitch, volume or filter, and a monophonic glide function (on some instruments, it was polyphonic).

Ian