  We present an explanation of the dynamical in-gap spin mode in LSCO induced
by an applied magnetic field H as recently observed by J. Chang et al. Our
model consists of a phenomenological spin-only Hamiltonian, and the softening
of the spin mode is caused by vortex pinning of dynamical stripe fluctuations
which we model by a local ordering of the exchange interactions. The spin gap
vanishes experimentally around H=7T which in our scenario corresponds to the
field required for overlapping vortex regions.
