A saturation dip has been observed recently by A. Kumar et al. [Chem. Phys. Lett. 297 (1998) 300] in degenerate four-wave mixing (DFWM) and two-color resonant four-wave mixing (TC-RFWM) spectra of jet-cooled CH. This observation suggests that expressions of the signal intensity established for double-resonance molecular spectroscopy in the weak held limit are no longer valid. While the line splitting has been explained, this is not the case for the asymmetry observed on this spectrum. Here, we clearly establish the origin of this asymmetry as resulting from the transverse molecular velocity which acts even for a small velocity component. The model conveniently reproduces line splitting and asymmetry.