We first saw this idea used at Willow Creek - it may have originated before that - and the YFC team used it really effectively last week at New Wine.
It is a way of helping people share what God has done in their lives in a simple but powerful way. People are asked to write on one side of the card a situation or state they were in before God intervened, and then on the other side where they are at now. They should attempt to do this in as few words as possible - for example: broken/healed, lost/found, friendless/friend of God, wandering/purposeful etc. Then people line up in front of the congregation and read out the first side, then turn it over and read the second. We interspersed this with the singing of a worship song like 'How great is our God', 'How great thou art' or 'Awesome God', so that God gets all the glory for the transformations.
The team also developed the idea, by giving everyone a piece of paper and allowing them to write their own testimonies, or difficult situations they are in and then on the flipside the positive outcome they would like to see. Obviously this needs to be handled sensitively as some people may have hard things going on which they feel are a long way from being resolved.