Last week, we worked the basic set up, controls, and finish for the Williams Guard. As I said before, this is a highly effect guard for either No Gi or MMA (works well Gi too) 
However, like any position in Jiu Jitsu, there are a number of defences to the Williams guard. 
Typically, your opponent will try to either step over your body or square up to you. While these are the correct defences to the Williams Guard, they can all be countered. Everyone knows only bad things will happen if you post on the ground. 
However, this is one of the common ways your opponent will try to defend the Williams Guard. 
Unfortunately, for them, this reaction will give you the perfect opportunity to set up the triangle.