You can program the low voltage using the ESC setup and your transmitter, but it's much better to have the castle link, because you will be able to customize your exact voltage cutoff (I believe I'm running mine at 6.6v which is very safe for my pack). With the transmitter and manual update, you are limited to "3v/6v/9v, etc..." I believe. With castle link you can also customize brake and throttle curves, etc...I noticed a big difference in both my setups after running the castle link and setting a few options that I'm not aware of being available by manually updating with the transmitter.
Castle link plugs into your esc through a usb and adapter. It's really simple software and it is very powerful in what you can do to customize your ride.
Trust me, for the money you will be thankful you purchased it. I would never sell my castle link, just because it is so handy. I use it quite often.