“Push the envelope” is used figuratively to mean “stretch the boundaries”. The image is not of pushing a mailing envelope across a desk: those who push this sort of envelope do it from within.
The phrase has its origins in the world of aviation, where ‘envelope’ has, since at least the late 60s, had the meaning ‘a set of performance limits that may not be safely exceeded.’ Test pilots are often called on to ‘push’ a new aircraft’s performance envelope by going beyond known safety limits, as in determining just how fast an airplane can be flown. In 1979 Tom Wolfe’s best-seller ‘The Right Stuff’ vividly described the life of test pilots during the 50s and60s, and it appears that this book, and the subsequent movie, did much to popularize the notion of pushing the envelope.
Opal Odyssey’s extension Push the Envelope deals with the recent socio-political happenings around the World, and discusses ways and means to attempt to change certain over-bearing systems and practises which have done much to hinder the progress, peace, and prosperity of Nations.