I think you might be referring to the "pass-along" savings cards that went out to current AP holders. Current AP holders were mailed a card that they can pass along to a friend. If the friend the purchases a new annual pass with it they will get 10% off the price. The person who gave you the savings card gets a $25 gift card for each annual pass bought. The only other stipulation is that the new annual pass must be purchased at the parks (can't use it for an exchange certificate) by July 31. You can use it if you plan on upgrading MYW tickets to an annual pass by then.
If you are going to be buying an annual pass by July 31 and this is what you will be doing and want a savings card send me a PM. I've got a few cards left and will mail them out to you. Each card is good for up to 6 six annual passes.