The progenitors of cataclysmic variables (CVs) undergo a common envelope phase, during which their initial orbital period is reduced to a few hours. After this phase the radiative core of the secondary might rotate at a lower rate than the tidally coupled envelope. A shear is then generated at the core-envelope interface which can give rise to a boundary layer dynamo and, consequently, to the magnetic braking of the binary system. The temporal variation of the shear energy content can be obtained by comparing the rate of magnetic energy production with the input rate of orbital energy in the convective envelope. We assume for the flux tubes a filling factor, fV, owing to the expected size of the tubes and the fact that the dynamo action may be concentrated inside an equatorial belt of the boundary layer shell. We also assume that the production of toroidal field is efficient only inside the boundary layer, while the poloidal component of the field can be enhanced also across the outer convective envelope (see also Zangrilli & Bianchini 1995). We find that the decrease in the shear energy, caused by the continuous production of magnetic field and by the progressive reduction of the core moment of inertia, leads the core coupling with the envelope, thus switching off the dynamo at the core-envelope boundary. Further details of this core-envelope decoupling dynamo model will be given in a forthcoming paper (Zangrilli, Tout & Bianchini 1996).