Yep it is from rugby league. I remember being being taught this move playing juniors in the mid-late 2000s. Assuming that the blocker was running an unders line back towards the ball player to give the option of a short ball rather than going around the back every single time, we called it a ‘B-block’ (I can’t remember what or if there was an ‘A-block’).
A ‘double block’ was a repeat sequence of the move and was really the only way to run it if you had a full line.
An interesting new rugby league move we may be seeing in rugby soon is the ‘slingshot’, great to watch
*Actually now that I think about it, ‘A-block’ might have been the standard block play as above and ‘B-block’ was the blocker running a tighter unders line to get the short ball from BC
