I really like this feature. Ensoniq Mirage’s very basic built-in sequencer does this too. And since its sequencer can have a few patterns in memory & stored to disk, it can be fun for live performance.

It’s a nice way to enter the notes you want, but have their rhythm remain undetermined/TBD.

Now that I think about it, it’s really pretty hard to replicate this when it’s not specifically supported.

JP-8000 mimicked this behavior over MIDI or local keys. It allowing incoming notes or the lower keyboard split to act as trigger. So you could hold keys on the right split but play their rhythm with the left, or send incoming notes on like channel 11 to trigger the keys you’re actually holding. (don’t quote me on that)

Oberheim Matrix-6/6R also has a trigger input that conventionally would be used for a sustain pedal, but can also be used to reset envelopes & LFOs, as mentioned above by @matthewsavant and @Microtribe et al