Historically, teamkilling in Siege has been handled by kicking a player after they’ve killed two teammates. Under the new system, there would still be amnesty for players who have killed a single teammate, but once that’s happened, they’ll absorb all the damage they do (or attempt to do) to their allies.
This allows for the accidental teamkill, which can easily happen in tense moments or when things get particularly chaotic. But Ubisoft wants to limit the ability of a single player to ruin an entire match by eliminating two members of their team before getting kicked out.
The idea here is that Ubisoft wants Siege to remain dangerous and risky, and for players to have to stay conscious of each others’ positions and lines of fire. This new system will limit the ability of teamkillers to exploit that danger, since after one friendly fire incident, they’ll be the ones getting damaged.