[NTG-context] Zoom image in presentation

Hello all, I'd like to be able to zoom an image during a presentation. My thinking is that it might be possible by turning layers on and off interactively. I see in this message: http://archive.contextgarden.net/message/20110426.183109.fc46c5a5.en.html that Hans said (enigmatically :-)) "And then there are viewerlayers (acrobat) that you can turn on and off with \goto{On}[VideLayer{mylayer}] but that's a different topic." Any hints on how I could do this? I'm using the latest standalone, installed yesterday. Thanks, Roger