I do this LOADS, but I use it a lot more to cover set ups for tricks.
For example, when I set up for the 3 card monte (cant reveal anything about the setup - but if you know the trick, you will understand) or even doing a get ready for a d/l I begin to say things about how easy it is to psycologically influence people (or whatever pater falls in with the certain trick) - eg, ID, NEVER state anything about the deck, or magical moves - all to do with influencing / subliminal messages - then be REALLY blunt, eg 'imagine you are the KING of the world, think of a card close TWO your HEART' etc, always gets a laugh, then you can say how you used the false messages to influence them to do a certain thing (again, in line with the given trick). Now you can do the trick, and as long as you kept eye contact, you could doing anything with your hands, and they wont have noticed (this was mentioned in an earlier thread by seige I think, mentioning about the importance of misdirection).
You can use this time to prepare your gimmick / any prop arrangements or setups that you may need, without anyone seeing
Hope this helps, its kinda reminded me that I need to write myself some new 'influential' patter for my newly learned tricks - simple card sleights from ellusionist (Dont hate me...they are cheap...

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Dan