The formation of membrane tubes (or tethers), which is a crucial event in many biological processes, is intrinsically a dynamic process. In this paper, we discuss both theoretically and experimentally the dynamical laws that govern extrusion and retraction of tubes extracted from lipid vesicles at high speed and under strong flows. A detailed description of the tether shape provides the first evidence that the tension along the tube increases from the vesicle body to the tip of the tube, while the tube radius decreases. As the pulling force is suppressed suddenly, the tube can relax only from the free end, and the velocity of retraction is a direct measurement of the frozen tension along the tube. We also report experiments on tethers pulled out either by mechanical point-forces or by hydrodynamic (electroosmosis-induced) flow, and we show that the observed dynamical laws for retraction are in good quantitative agreement with our theoretical predictions.