The Fresnel biprism is a simple prism, but with a large apex angle. It refracts the upper half of the beam downward and the lower half upward, crossing them in the crystal at an angle. The result is that the delay between the two crossing pulses varies along the transverse direction (x). So we need only image the crystal onto a camera to obtain an autocorrelation on a single shot.
To make a FROG, we must also spectrally resolve the resulting second-harmonic pulse. And the thick crystal does this for us! The thicker the crystal, the narrower the range of wavelengths emitted in a given direction, so a thick crystal naturally spectrally accomplishes this! Focusing this light onto the camera maps wavelength to the other transverse direction (y) and so yields a single-shot FROG measurement.
Here are two views of GRENOUILLE, one from the top and the other from the side. It's the simplest pulse-measurement device ever invented!
Here's the first demonstration of GRENOUILLE, by grad student Patrick O'Shea. Notice that its measurement of a somewhat complex pulse agrees with a FROG measurement of the same pulse.