The best description I have seen of it is in Wolfgang’s message on the mailing list at http://www.mail-archive.com/ntg-context%40ntg.nl/msg77942.html (or http://www.ntg.nl/pipermail/ntg-context/2015/081569.html if you prefer pipermail navigation). Basically, define the spacings you want with \definecharacterspacing[myspacing][…][…=…,…=…] and put them in play with \setcharacterspacing[myspacing]. Do it within a group to have it apply only within that group, or apply it globally.
Is there any place I could find docs on \definecharacterspacing command please ? Could not find any occurrence on the Wiki.