Vincente Junior shows us some modifications to the classic arm bar from guard that he likes to use to isolate the arm he's attacking. Catching someone with an arm bar if they've been training for a while can be difficult, most people learn relatively quickly how to defend against it. These modifications might be what you need to put this iconic attack back into your bag of tricks.
There are several ways and grips to set up the Peruvian Necktie. This is how we do it in the 10th Planet Jiu Jitsu system. You can't have your hands too tight or they will pop off when you sit back and throw the leg over. This is a very tight choke which comes on in less than 3 seconds.