  Relativistic magnetohydrodynamics can be reformulated in terms of magnetic
flux tubes which turn out to obey equations of non-linear strings. The string
approach is applied to study how a test flux tube falls into a Kerr black hole.
Analytical treatment and numerical simulations show that the leading portion of
the falling tube loses angular momentum and energy as the string breaks, and to
compensate for this loss, momentum and energy has to be generated to conserve
energy and momentum for the tube. Inside the ergosphere the energy of the
leading part can be negative, and the rest of the tube then extracts energy
from the hole. Increasing centrifugal forces eject the part of the tube with
extra positive energy from the ergosphere after a time producing a relativistic
jet.
