A partial agonist binds a receptor producing only a partial response (green dashed line) in comparison with a full agonist (black line).
When partial agonist is added to the full agonist in small amounts, the response is higher than that of the full agonist alone (blue line, low concentration). The partial agonist occupies the receptor binding sites that were not taken by the full agonist and thus has an additive effect.
However, at higher concentrations, the partial agonist occupies binding sites that normally would have been occupied by the full agonist at high concentrations. So, in the presence of a full agonist, the partial agonist reduces the maximal response. Thus the partial agonist actually acts as a competitive antagonist.