[edited from support email]
FYI, the onset parameter is listed in the users guide as a parameter but there is no explanation of how to use it! We will fix this but in the meantime, the parameter simply causes a delay in milliseconds in terms of when the question wording appears relative to the rest of the item. So adding o750 will cause the question wording to appear 750ms from the start of the item. It's useful for creating a visual break between items--especially when many scale items are being answered back to back.
Note that reaction times do not take onset into account--that is something you need to deal with in your analyses--all RTs begin from the initial appearance of any given item.
Blair