That transfer became a free transfer when they moved things around and didn't allow for transfers anymore at the 53rd street station from the F to the 6 lines (when the V started running in 2001).

To keep all those people from rioting or whatever, they allowed them here, under the condition you swipe your metrocard within two hours of the first swipe.

I'm not sure if the first swipe has to be at an F train station or any station, but do know there's a two hour window.