Spectating is watching other players play by choosing "Spectate" in the options menu. You will not spawn and your screen will be where the 1st cell by the number of their mass is. You are also able to move around by pressing "Q" and using your mouse.
You can use spectate mode for this troll trick:
1. Go on spectate mode.
2. Look for a team.
3. Wait till one teammate dies.
4. Name yourself the name of the eaten teammate!