The SATR probes take advantage of the high refractive index of sapphire to consistently generate total internal reflection at the interface between the probe and the sample. When a beam is reflected through total internal reflection, an evanescent wave with the same energy is generated. An evanescent wave is basically an electromagnetic wave that doesn’t propagate, but can still interact with matter.
This evanescent wave only penetrates the sample a few micrometers. This is essentially a microscopic path length, which is useful for absorption spectroscopy of samples that would saturate a detector otherwise (for example, if the sample is highly concentrated or has a large molar attenuation coefficient). You get an absorption experiment, the same as you would with a cuvette and a transmitted beam, but with a microscopic path length and a reflected geometry that ends up being more compact. SATR probes are great for inks, dyes, and other samples that are very concentrated and the user wants to test their sample without dilution.
