Confinement of light by subwavelength objects facilitates the realization of compact photonic devices and the enhancement of light-matter interactions. The Fabry-Perot (FP) cavity provides an efficient tool for confining light. However, the conventional FP cavity length is usually comparable to or larger than the light wavelength, making them inconvenient for many applications. By manipulating the reflection phase at the cavity boundaries, the FP cavity length could be made much smaller than the wavelength. In this review, we consider the subwavelength FP resonance in a plasmonic system composed of a slit grating backed with a ground plane, covering the spectral range from microwave to THz and infrared regime. For very narrow slit width and spacer thickness, a typical zero-order and deep subwavelength FP resonance in the metallic slits can be strongly induced. Moreover, due to the subwavelength FP resonance, greatly enhanced electromagnetic pressure can also be induced in the system. The sign and magnitude of the electromagnetic pressure are dominated by the field penetration effect in the metal as well as the field enhancement in the FP cavities. The effect promises a variety of potential applications, such as detecting tiny motions and driving the mechanical oscillations.