  We investigate the formation of photonic bound states in the continuum (BICs)
in photonic crystal slabs from an analytical perspective. Unlike the stationary
at-$\Gamma$ BICs which origin from the geometric symmetry, the tunable
off-$\Gamma$ BICs are due to the weighted destructive via-the-continuum
interference in the vicinity of accidental symmetry when the majority of the
radiation is pre-canceled. The symmetric compatible nature of the off-$\Gamma$
BICs leads to a trapping of light that can be tuned through continuously
varying the wavevector. With the analytical approach, we explain a reported
experiment and predict the existence of a new BIC at an unrevealed symmetry.
