We have two dogs and bought the collar that could control two collars. My dogs growl at each other and so I wanted to be able to switch. Ack and forth from dog to dog from one controller easily. This is no a quick and easy function with this receiver. You have to circular rotate one knob while pushing another button. This controller is capable of controlling 4 collars. So it is hard to just stay between dog one and dog two while trying to go back and forth for training. Same with changing the level of shock and vibration. It isn’t a quick transition. The vibration is strong. The shock is strong. It can go up to level 100 shock. I tried one on my leg and it hurt pretty bad. I actually never shocked my dogs but I was not worried about the shock being able to go through their thick, furry, fatty necks. Definitely start low with the shock and work your way up. I couldn’t believe how bad level one hurt when we tried it. I am not sure if we will be keeping this. With practice I’m sure changing the level of shock strength and switching dogs would become easier.