As a refresher, Teenage Engineering is famous for off-beat synthesizers that marry Swedish design with pouting EDM-style sounds and quirky extra features like video output. Most of the things the company makes are portable, and have either a tiny speaker, or require the use of headphones. Basslet, is a wearable "tactile" subwoofer that translates bass sounds into something you can feel. Putting the two together is a logical move.
Basslet will already work with anything that has a 3.5mm audio port. The problem is, you'll always need Basslet's companion dongle -- which adds another link in the chain (plus it needs charging). What Teenage Engineering will do with the OP-Z, is bake the dongle right into the synth, or at least the required wireless connectivity, so you can just plug your headphones in and play.
I had a quick demo on a very (very) noisy trade show floor, and the benefit was instantly obvious. What would otherwise have been a hard-to-hear beat suddenly leapt out, with every kick drum enhanced by the thudding Basslet on my wrist. Making music on the move will become a much more immersive experience, and not having to pack that pesky dongle will really be handy. Generally in life, we need less dongles, this much we already know (someone please tell Apple).
The bad news is, the OP-Z isn't slated to be released until fall this year.