  We consider quantum transmission through double-bend $\Pi$- and $Z$-shaped
waveguides controlled by the finger gate potential. Using the effective
non-Hermitian Hamiltonian approach we explain the resonances in transmission.
We show a difference in transmission in the short waveguides that is the result
of different chirality in $Z$ and $\Pi$ waveguides. We show that the potential
selectively affects the resonant widths resulting in the occurrence of bound
states in the continuum.
