  We describe the spontaneous partial breaking of $N=1 D=10$ supersymmetry to
$N=(1,0) d=6$ and its dimensionally-reduced versions in the framework of the
nonlinear realizations method. The basic Goldstone superfield is $N=(1,0) d=6$
hypermultiplet superfield satisfying a nonlinear generalization of the standard
hypermultiplet constraint. We interpret the generalized constraint as the
manifestly worldvolume supersymmetric form of equations of motion of the Type I
super 5-brane in D=10. The related issues we address are a possible existence
of brane extension of off-shell hypermultiplet actions, the possibility to
utilize vector $N=(1,0) d=6$ supermultiplet as the Goldstone one, and the
description of 1/4 breaking of $N=1 D=11$ supersymmetry.
