Push the Envelope

I was thinking to myself today, wondering the meaning of the phrase 'To Push the Envelope'.

Any personal risk resulting from action is usually referred to as pushing the envelope. Considering this, when the phrase is mentioned I naturally think to myself that to push the envelope means to force the flap away from the sticky residue holding it shut. So if we can safely assume this, might we also assume that whatever is inside the envelope itself, contents tangible or otherwise, might be something similar to Pandora's Box?

Perhaps it's safe to assume that the ultimate risk would be actually Opening the Envelope.