  An identity is derived from Einstein equation for any hypersurface H which
can be foliated by spacelike two-dimensional surfaces. In the case where the
hypersurface is null, this identity coincides with the two-dimensional
Navier-Stokes-like equation obtained by Damour in the membrane approach to a
black hole event horizon. In the case where H is spacelike or null and the
2-surfaces are marginally trapped, this identity applies to Hayward's trapping
horizons and to the related dynamical horizons recently introduced by Ashtekar
and Krishnan. The identity involves a normal fundamental form (normal
connection 1-form) of the 2-surface, which can be viewed as a generalization to
non-null hypersurfaces of the Hajicek 1-form used by Damour. This 1-form is
also used to define the angular momentum of the horizon. The generalized
Damour-Navier-Stokes equation leads then to a simple evolution equation for the
angular momentum.
