Tier 1 - Downed crawl speed is increased by 100%
Tier 2 - Reduce the delay before health starts regenerating by 50%
Tier 3 - reviving and ally will also heal you to full health
Tier 4 - Reviving an ally will increase both players movement speed for a limited time
Tier 5 - Killing an enemy while downed will revive you and remove Quick Revive Perk