The optical centrifuge was demonstrated in 2000 as a tool for preparing ensembles of molecules in extreme rotational states. Highly rotationally excited molecules, so-called superrotors, are observed as products of photodissociation and molecular collisions, in high-temperature environments in the atmospheres of Earth and exoplanets, and in the interstellar medium. Traditional optical excitation is limited to small changes in rotation, limiting experiments to relatively low rotational states. In this review, I discuss the use of a tunable optical centrifuge to prepare molecules in selected ranges of excited rotational states and investigations of their collisional relaxation using state-resolved polarization-sensitive transient IR probing. I examine the decay dynamics of population, alignment, and translational energy release, focusing on experimental results, and compare them with simulations that overestimate observed relaxation rates. A clear picture of near-resonant and nonresonant energy transfer pathways emerges and establishes the means to distinguish superrotor and bath collision products.