Rush bombs don't detonate if someone is actively defusing.

If someone's hand is currently on the bomb, defusing it, then the bomb will not detonate, even if the 'timer' reaches 0. This means that reaching the bomb in the second will not end with the bomb saying 'ha ha try again' and blowing up. Instead, it will remain intact until you either finish the defusal(?), where it will become dormant once more, or until you stop defusing, in which case the bomb will explode.

To be clear, this is not a suggestion to 'pause the timer' while the bomb is being defused. If the timer is on 20 seconds and you start defusing the bomb but get killed after 4 seconds, the timer will continue from 16 seconds.