If you already have a multi-day pass or annual pass, you can exchange your voucher for a special Fast Pass instead. While there haven't been any first hand reports yet on how this will work, many think it will be very similar to last year's birthday fastpass option. For one GAD voucher at any park, you will receive special fastpass cards for you and up to 5 others in your party. There are special fastpass machines at the specified rides that will take these cards and give you a FP for a window that begins immediately.
I'm planning to use this option. My sister & I are going in March, so we will have two vouchers. We'll use one in the MK, but I'm not sure whether to use the other one at Epcot or AK. Either way, I'm sure it will help save time since we'll be going during the spring break crowds.
ETA: I stand corrected. There are several first hand reports from people who have used the FP. I just had not read that far yet!