The Windows OS has a mechanism that allows to notify a window when the content of the clipboard changes.
Not just one window can be notified but many. They are organized in a chain. The first window in this chain receives a notification and passes it to the next one, so the notification goes down to the last window registered in this chain.
Any FoxPro window, the _screen or a form, can be registered as a Clipboard viewer -- this is how it is called. The notifications are actually window messages. And VFP9 handles window messages really good due to extended BINDEVENT() function.
WM_CHANGECBCHAIN -- this notification is sent after a registered window has been removed from the chain of clipboard viewers.
WM_DRAWCLIPBOARD -- this notification is sent after the content of the clipboard has changed.