  Aims. We introduce a new method to calculate and interpret indirect
transition rates populating atomic levels using Markov chain theory. Indirect
transition rates are essential to evaluate interlocking in a multi-level source
function, which quantifies all the processes that add and remove photons from a
spectral line. A better understanding of the multi-level source function is
central to interpret optically thick spectral line formation in stellar
atmospheres, especially outside local thermodynamical equilibrium (LTE).
Methods. We compute the level populations from a hydrogen model atom in
statistical equilibrium, using the solar FALC model, a 1D static atmosphere.
From the transition rates, we reconstruct the multi-level source function using
our new method and compare it with existing methods to build the source
function. We focus on the Lyman series lines and analyze the different
contributions to the source functions and synthetic spectra. Results. Absorbing
Markov chains can represent the level-ratio solution of the statistical
equilibrium equation and can therefore be used to calculate the indirect
transition rates between the upper and lower levels of an atomic transition.
Our description of the multi-level source function allows a more physical
interpretation of its individual terms, particularly a quantitative view of
interlocking. For the Lyman lines in the FALC atmosphere, we find that
interlocking becomes increasingly important with order in the series, with
Ly-{\alpha} showing very little, but Ly-\b{eta} nearly 50% and Ly-{\gamma}
about 60% contribution coming from interlocking. In some cases, this view seems
opposed to the conventional wisdom that these lines are mostly scattering, and
we discuss the reasons why.
