From what I understand/remember an example:
Player 1(with 220ms ping) shoots player 2 at 00:30.00, player 2(with 50 ping) also hits player 1 at 00:30.10, player 1 dies first at 00:30.15, without trade kills player 1 who hit player 2 first would not get the kill even though they hit player 2 first, with trade kills player 1 would get the kill as well even though from the server's perspective that player is dead.
00:30.00 Player 1 Hits player 2.
00:30.10 Player 2 hits player 1.
00:30.15 Server knows player 2 hit player 1 and player 1 is dead.
00:30.22 Server, player 1 is dead so hit is ignored.
I'm probably wrong though...
There was a fun bug you could do with newnet years ago not sure if it's still possible, if you set your ping really high like 1300ms hit a person twice in combogib they would die, and if they respawned fast enough your second hit would count even though they respawned, they would die again even though they could be on the complete opposite side of the map.