[NTG-context] Presentations in context: Is there a reference manual?

Hi! Is there any reference manual for doing presentations with context? I only got art-pres.pdf but it doesn't give many details. What is the purpose of \presentationstep? My first attempts using context didn't show great diferences with and without this command. Is it possible to define timings for automatic transitions between slides? Best regards, Luís Neves