For a hardfork to take effect, 17 / 21 of the witnesses need to vote 'yes'. The 21 witness votes come from the 20 top witnesses + 1 backup.
Backup witnesses do get a 'say'. They can contribute to the discussions, and express the way they plan to vote.
If a backup witness is voting in a way that the stakeholders like, and the top witnesses are voting in a way that the stakeholders don't like - then the backups may be voted in as the new top witnesses.