The Enketo blog [http://blog.enketo.org/] introduces this new feature.
Here is a screen grab -
How to Use
No need to add anything to your XLSForm. You have the following options to use this feature for any form:
A. Be an activist
You could actively request any Enketo-powered service to leverage this new feature. Those services rely on your feedback to continuously improve.
B. Use your own Enketo Express installation
How would I go about getting this activated on KoboToolBox using option A above?