If Kecleon would be, for example, using Flamethrower, before to attack, it would turn to a Fire type Pokemon and get STAB.
Entry: Changes the Pokémon's type to the type of the move it's using.
In-Depth Effect: Before the Pokémon uses a move, it becomes a pure Pokémon of that type.
Here is the source. Yay ! I know now how to "change the name of a link" ! It's by Hyperlinking !
Hope this help :)