  In a chain of mutually coupled oscillators, the coupling threshold for
synchronization between the outermost identical oscillators decreases when a
type of impurity (in terms of parameter mismatch) is introduced in the inner
oscillator(s). The outer oscillators interact indirectly via dynamic relaying,
mediated by the inner oscillator(s). We confirm this enhancing of critical
coupling in the chaotic regimes of R\"ossler system in absence of coupling
delay and in Mackey-Glass system with delay coupling. The enhancing effect is
experimentally verified in electronic circuit of R\"ossler oscillators.
