Ben Vallack introduces his 16-key layout along with its evolution in a nicely edited video:
In this video I go into some detail about the layout I'm using on my 16 key keyboard. It has taken me a while to get to this point and I think there are now very few barriers to this keyboard being able to sustain pretty fast typing – Ben Vallack.
The main thing that makes this layout so unusual is the way Ben deals with using two layers for the alpha keys. This issue is more complicated than it first seems due to needing to use shifted and non-shifted versions of keys on both layers.
Ben thinks he have eventually come up with a mechanism that seems to work pretty well.
Felix Kühling explains how hexagonal keys work surprisingly well in an ergonomic layout without fine-tuning parameters like hand rotation, column stagger and splay.