We employ the free field realisation of the
world-sheet theory to constrain the correlators of string theory on
{\rm
AdS}_3\times {\rm S}^3\times \mathbb{T}^4
|
with unit NS-NS flux. In particular, we directly obtain the unusual delta function localisation of these correlators onto branched covers of the boundary
by the (genus zero) world-sheet -- this is the key property which makes the equivalence to the dual symmetric orbifold manifest. In our approach, this feature follows from a remarkable `incidence relation' obeyed by the correlators, which is reminiscent of a twistorial string description. We also illustrate our results with explicit computations in various special cases.
