I have been playing for honour again recently and for the past week i have been taking 100% of all damage from everyone in ganks. Meanwhile my 35 damage heavies get dropped to 7 damage because my teammate was having a staring contest with the enemy. I am getting guard-broken or get put in recovery and the 3 people can 1 shot me from full health.
Damage reduction occurs in a variety of different ways and there are moves that trigger damage reduction and moves that do not. Moves like Gladiator's toe stab, Centurion's ground pin and Warlord's crashing charge do not trigger damage reduction for example.
On moves that do trigger damage reduction such as Shaman's bite, regular attacks and most bashes: if you're hit by more than 1 external attacker at the same time a 70% damage reduction is applied to the 3rd person that hit you.
Damage reduction on counter-GBs and hit stun from bashes has a specific timing window where it triggers. This can be bypassed by players doing a co-ordinated gank and hitting on the correct timings to avoid damage reduction.