What happens is this reaction momentarily binds to the surface of our catalyst and this weakens the double bond. And then this other reactant can come from the top, attacking these carbons, thereby creating our product. Now, this is how metal catalysts act. An example of such a metal catalyst is, for example, fuel cells. In fuel cells, plant and catalyst acts in the same manner to speed up the reactions the oxidation and reduction reactions in an anode in a cathode. Now, if you want to learn more about fuel cells, check out the link above.