shooting aid????

[paulaboutform]

A few years ago at a national 3D championships it seemed almost all the xbow shooters had these funny rubber looking conglomerations added to their forestocks. This was a a Canadian National shoot in Barrie Ontario in case someone happened to be there and saw them. They all appeared to be custom to aid in aiming by tucking the arm into the body to stabilize the forestock. Anyway, if anyone knows what they are can you please tell me. I'm not quite sure how else to describe it but if you've seen it you know what I'm talking about. Thanks for the help.