This is run on DFU Mode and loads a secondary DFU-like interface. WTF Mode stands for "What's The Firmware" according to PurpleRestore and a quick look at some internal binaries.
This doesn't appear to do anything. It calls the same functions in the VROM that the VROM calls to run the DFU. The only observable difference is that this WTF is slower. This may also allow large files to be uploaded.