The figure shows possible migration between a place of origin and a place of destination,with positive and negative signs signify pull and push factors, respectively. Flows take place between two places, but there are intervening obstacles to these spatial movements. Although these obstacles are represented by "mountain" shapes, keep in mind that the obstacles need not be limited to physical barriers. Restrictive immigration laws, for example, can present a formidable barrier to prospective migrants. Note that both the origin and destination have pushes and pulls, reflecting the reality that any migrant must consider both the positives of staying and the negatives of moving, as well as their converses. The logic of the push-pull theory is that if the plusses (pulls) at the destination outweigh the plusses of staying at the origin, as shown below, them migration is likely to occur. 

Márcio Domingues