You can activate BackUp in your cell at the cost of one death. After activating it, "you can help superior teammates in fights". This does rest for 24h.
This means the brute who has this ability activated will join the fights of the other brutes on the same account that are at a higher level, for 2-3 hits. It doesn't work in tournaments.
Really good skill if you have more than two brutes on your account. Then you can just stay on the same level with your support brute and easily level up the others!