One of the factors that the collision theory encompass to explain the rate of reactions is the orientation of the collisions.
In some reactions the only some special orientations yield effective collisions (those which will result in a chemical change).
In the case of the ehtylene and hydrogen chloride, the hydrogen end of the hydrogen chloride molecule must collide with the double bond of the ehtene molecule, to transform the change the c-c double bond iinto a single bond.
This approach is represented by the collision 1 of the diagram.
For your information, when the Chloride end of the hygrogen chloride molecule approachs the double bond of the ethylene molecule, the excces of negative charge on the chloride (due to its electronegativity) will be repeled by the electron density on the double bond, and the molecules will bounce instead of react.
