Knots

[scoot]

Its just how the leaves were bunched by the roller. If there's too much filler in one particular spot its not going to allow for an open draw. However, if its just one spot and its close enough to the foot once you smoke through it the draw will open up. Its not an actual knot in the tobacco, but it would seem more probable that its simply your firend's way of labeling the occurrence I described.