The timpanist is "the second conductor"
The timpani is a vital pillar of the orchestra, and has been used effectively by numerous distinguished composers such as Mozart and Beethoven. When the sound of the timpani is heard in a song, it signals an important part of the music, so the timpanist must come in with accurate timing and expression.
So, what would happen if the conductor and the timpanist weren't in agreement on a certain musical passage? Being an instrument with great impact, the other players in the orchestra would likely fall in time with the sound of the timpani. For this reason, the timpanist is called "the second conductor".