The effect of the multidimensional character of methyl internal rotation on torsional potential functions in small conjugated methyl molecules is discussed. Partitioned energetics obtained from different ab initio calculation levels are divided into sigma and pi symmetry components and then further dissected by breakdown of the fully relaxed internal rotation process into separate relaxation steps and their nuclear virials. Important contributions to barrier shape, height, and origin are thus disclosed. The analysis reveals closely balanced antagonistic pi and sigma contributions as determinants for the barrier height and identifies the dominant contributions to the barrier origin as sigma-bonding changes largely resulting from bond lengthening during methyl group rotation. Skeletal hydrogen out-of-plane motion components of the torsional coordinate (e.g. CH2 ethylenic twisting in propene) provide a general mechanism for barrier narrowing in small conjugated methyl molecules. Rydberg jet experiments are discussed as a general means for obtaining accurate torsional frequency information.(including ground state) to test internal rotation potential surface models.