  We study the mechanism of directed energy transport for soliton ratchets. The
energy flow appears due to the progressive motion of a soliton (kink) which is
an energy carrier. However, the energy current formed by internal system
deformations (the total field momentum) is zero. We solve the underlying puzzle
by showing that the energy flow is realized via an {\it inhomogeneous} energy
exchange between the system and the external ac driving. Internal kink modes
are unambiguously shown to be crucial for that transport process to take place.
We also discuss effects of spatial discretization and combination of ac and dc
external drivings.
