Cheletropic reaction
In the pericyclic transition state, a small molecule donates two electrons to the ring. Two possible geometries can explain the reaction.  The small molecule can approach in either a linear or non-linear way. In the linear approach, the electrons in the orbital of the small molecule are pointed directly at the π system of the big molecule. In the non-linear approach, the orbital approaches at a slightly off angle. The π-systems ability to rotate as the small molecule approaches is crucial in forming new bonds. The direction of rotation will be different depending on how many π-electrons are in the system. A diagram of a two-electron fragment approaching a four-electron π-system using frontier molecular orbitals is shown below. The rotation will be disrotatory if the small molecule approaches linearly and conrotatory if the molecule approaches non-linearly. Disrotatory and conrotatory tell how the bonds in the π-system are rotating.  Disrotatory means opposite directions while conrotatory means the same direction. This is also shown in the diagram below.