Torsional strain or eclipsing strain is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom.
Consider two conformations of ethane:
The smallest dihedral angle is 60º in 1; it is 0º in 2. Thus, two C-H bonds not sharing a carbon atom that are closest to each other (ex: C-H1 and C-H4) are closer in 2 than in 1. The repulsion between electrons in bonds that do not share a carbon atom is, therefore, greater in 2 than in 1. Consequently, torsional strain is greater in 2 than in 1.