It forces a complete wireless synchronization between the device's email client and the server. I'm not sure that it actually has an effect unless you're on BES (BlackBerry Enterprise Server), usually found in a corporate environment. Here, it forces all messages to be delivered to both desktop and device, and forces synchronization of the message status (read, deleted, sent, etc).
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Exactly, I know sometimes when I hit it, the berry will update some of the differences from my Hotmail but not everything. Kind of annoying but oh well, a while ago most phones couldn't even pull web based email so things are good!!
Posted from my CrackBerry at wapforums.crackberry.com