Countdown = GEM saw you and will "explode" when it hits 0, taking out everyone in range. Protect yourself by (1) Being somewhere else when it 'splodes (2) Shooting the GEM before it 'splodes, or (3) standing directly underneath the GEM when it 'splodes (this makes a useful cover - during the second game I spent some time hanging out under a GEM near the middle of the arena, which occasionally took out people I missed :-).