One thing that BJJ does better than a lot of grappling arts is come up with unique names for moves. In Judo, most variations just get boiled down to one of the original moves and then it is just a variation of that move, even though the mechanics might be completely different (like a Koga Seio Nage). But sometimes in BJJ different people end up inventing the same move and name it different things. Then it becomes a little bit of a popularity contest to see which name wins out in the end.
There were a couple matches in 2005 that brought popularity to this guard and position. The biggest upset at the time was Demian Maia vs Jacare Souza. Jacare now more popular for his UFC career was the open weight world jiu jitsu champion at the time. Demian was a talented competitor, but was a clear underdog at the time. Demian with his new guard and offside attacks got the better of Jacare at the World Cup in 2005. He came out with a video series and named the position anaconda guard, so that was how I was introduced to the name and I stuck with it ever since.
The same year that Demian beat Jacare, Marcelo Garcia was continuing his tear in the ADCC no gi grappling world. A very fun match that I believe Marcelo credits to creating the single leg x guard was his match with Diego Sanchez in ADCC 2005. Marcelo had been setting up his famous x guard from the butterfly guard, but against standing opponents it was harder for him to enter. This is the first time I believe he used it in competition. He didn't popularize it as single leg x guard until his technique website MGinAction.com came out in 2010.
Also in ADCC 2005 Xande beat ultra heavyweight world champion and future UFC heavyweight contender Gabriel Gonzaga. Xande used the same guard to sweep Gonzaga and finish the match in under a minute. We used to just call the position footlock guard because you can get to the anklelock and other footlocks from the position.
In the end, Single Leg X (often abbreviated as SLX) won the popularity contest probably because of the growth of Marcelo Garcia's popularity (and known as all around nicest guy in BJJ). I was slow to adapt it as the name because I had heard about it as anaconda guard for years. The benefits for calling it Single Leg X over anaconda guard are I think people gain a familiarity with the X guard position and it links itself to the x guard and in practice you are going to bounce back and forth between x guard and single leg x guard all the time. I think a new practioner benefits from seeing this relationship in the name of the guards. Also there is a mental boost from that if you good good at single leg x, you will probably see it as not a big of jump to learn x-guard. Plus the famous arm-in chokehold the anaconda choke doesn't have any relationship with the anaconda guard so having the same name to the choke is a bit of a disservice.
In terms that I hate to use, because of EBI there is a rise in the popularity of heel hooks and one of the innovators is John Danaher from Renzo Gracie academy. Unfortunately he uses the Japanese terms for the position such as ashi garami. Which doesn't help the american practitioner learn the move an easier because most don't speak that language. You could argue that the foot position of the inside leg is different than single leg x because it is on the far side, but that is the usual position that it falls into during the sweep anyway.
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