  By computing the completely bounded norm of the flip map on the Haagerup
tensor product $C_0 Y_1\otimes_{C_0 X} C_0 Y_2$ associated to a pair of
continuous mappings of locally compact Hausdorff spaces $Y_1\rightarrow
X\leftarrow Y_2$, we establish a simple characterisation of the Beck-Chevalley
condition for base change of operator modules over commutative $C^*$-algebras,
and a descent theorem for continuous fields of Hilbert spaces.
