We discuss the method to investigate the origin of hadron resonances from the analytic structure of the scattering amplitude. Recently, there have been discussions on the relation between the hadron structure and the position of the CDD (Castillejo--Dalitz--Dyson) zero, which is defined as the zero of the amplitude. To study the relation between the position of the CDD zero and the internal structure of the hadrons, we consider the zero coupling limit (ZCL) of the coupled channel scattering amplitude, where the couplings among different channels are turned off. With the behavior of the pole in the ZCL, we reveal the origin of the eigenstate.
By studying the behavior of the eigenstate poles and the CDD zeros in the ZCL, it is shown that the origin of the pole accompanied by a nearby CDD zero is the dynamics of the hidden channel. As an application, we show that the High-mass (Low-mass) pole of the $\Lambda(1405)$ baryon has the dynamical origin of the $\bar{K}N$ ($\pi\Sigma$) channel.