  We employ the free field realisation of the $\mathfrak{psu}(1,1|2)_1$
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 ${\rm S}^2$ 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.
