You have selected the following FlashKernel for Mobile ODIN:

Sprint Galaxy S4 SPH-L720
v6.0, based on MobileODIN System v3.90

After clicking download, your device should start downloading the file. When the download is complete, you need to open the file to start installation.

FlashKernels are special kernels used by Mobile ODIN to help flashing the selected firmware, if it cannot be done with the kernel you are currently running. The FlashKernel is used temporarily, it does not remain on the device after the firmware flash is complete.