EnterBL.exe is a small program that is part of the ROM update or install package. It ensures your device enters Boot Loading Mode prior to starting the actual ROM upgrade.
See for more info BaUpgradeUt.
How it works
First, EnterBL calls KernelIoControl(0x01012c04); ( 'RUU-Write-Flag' ) and then resets the device. Then, the bootloader then recognizes the flag written by KernelIoControl(0x01012c04), and boots into the bootloader mode, instead of Windows CE.