One of the main results of the Wendelstein 7-AS stellarator (major radius2 m, average plasma radius 0.18 m, magnetic field 2.5 T, low shear) is theachievement of the H-mode confinement in ECR- and NBI-heated plasmas. Thereis a strong dependence on the external rotational transform; the H-modeconfinement can be found only in a narrow window close to ι = 1/2.Viscous damping and the interaction with a neutral background are the onlydamping mechanisms proposed so far, which inhibit the poloidal shear flow.Viscous damping is computed using collisional and weakly collisionlessapproximations (plateau regime). Large values of the poloidal viscosity arefound on rational magnetic surfaces, while in the neighbourhood oflow-order rational surfaces the viscosity is very small. Islands provide amechanism for enhanced momentum transport in the radial direction, whichleads to an effective shear viscosity. In Wendelstein 7-AS islands exist onthe `natural' rational surfaces with ι = 5/9,5/10,5/11,....Furthermore, a next generation of islands exist on ι = 10/19,10/21.Experimental results in Wendelstein 7-AS confirm the hypothesis that theH-mode only can arise if none of these islands exist in the plasma. Theregions of rotational transform predicted by this hypothesis roughly agreewith those of the experiment.The extrapolation towards Wendelstein 7-Xshows that a similar case is expected there. Numerical calculations ofislands are made in vacuum fields; the evolution of the islands with risingplasma pressure is unknown. Finally a qualitative model of H-modedevelopment is discussed.