I've always been wary of it, because followers occasionally enjoy standing in deadly deadly arcane orbs or some crap... so now you have an undying follower blasting you with arcane orb damage.
When your follower has the relic, and they get to 1 life, they stop taking damage. No damage taken = no damage to split = you take no damage from follower. Your damage is split to your follower who ... takes no damage. So its an effective 50% damage reduction once your follower reaches 1 life.
WD Season 8 https://www.diabloprogress.com/hero/finiar-1655/Kildare/84509816
Monk season 7 http://www.diabloprogress.com/hero/finiar-1655/MojoJoJo/42225505
DH season 6 http://www.diabloprogress.com/hero/finiar-1655/DeadShot/75655606
Angry Chicken http://www.diabloprogress.com/hero/finiar-1655/WhoDoVooDoo/68187610
What? Me worry?
WD Season 8 https://www.diabloprogress.com/hero/finiar-1655/Kildare/84509816
Monk season 7 http://www.diabloprogress.com/hero/finiar-1655/MojoJoJo/42225505
DH season 6 http://www.diabloprogress.com/hero/finiar-1655/DeadShot/75655606
Angry Chicken http://www.diabloprogress.com/hero/finiar-1655/WhoDoVooDoo/68187610
What? Me worry?