Evolution of flux and size of four radio knots of BL Lac 5,6,7 and 10. The grey hatching area represents the duration of the perturbation propagation in the VLBI jet from the knot 10 to the knot 5 between the October 14th 2001 and the July 7th 2002. The non-observation of the knot 7 and the appearance of the knot 10 during the perturbation propagation is a sign of an abrupt evolution of the same knot.