While some inexplicable force will explain the crossover between the Street Fighter and Tekken universes, the same force will also inexplicably power-up your fighters for the win. Pandora's Box brings the fighters together, but Pandora Mode will give players the ability to quickly finish the match, but only at great risk.
Pandora Mode can be activated when one partner has less than 25% health. Activating the power will kill that character to give your other fighter unlimited EX meter… for only eight seconds. If you fail to defeat your opponent within that time frame, your second fighter will die. Capcom's Seth Killian says:
They basically sacrifice themselves to the Pandora Box, which loves fighting, and which has brought the Tekken and Street Fighter universes together. There's really no disadvantage to [X-Factor and other last-chance moves]. You get them for being beaten up, but there's no disadvantage. This has a massive disadvantage.