It consists of two rubber bumpers that stop the string from flexing h beyond the brace height that's set. Reduces vibration and helps prevent damage to bow if dryfired. e.g. http://www.outdoorsexperience.com/excalibursts.html
Now available for several models in addition to Excal. e.g Parker,
Difficulty of making them depends on the kind of bow you have (they are attached to the riser) and makers mechanical skills.