Thnx in advance

The best way to see this in action is to go to the calculator on Spirea's old page (link anyone?), but I'll try and explain it here:
All items and spells that use poison in the game are displayed as x-y Poison Damage over z Seconds, right? Well, the game actually handles is as MIN dmg/sec and MAX dmg/sec over z Seconds. Now, when you add poison things together (eg Poison Dagger with a Dagger of Pestilance) you add together the MIN dmg/sec and the MAX dmg/sec to get the total MIN and MAX dmg/sec...and you add the durations together to get the total Poison Duration. This means it's very easy for the poison to last for up to 30 seconds (perhaps more), but you are doing a lot of damage each second. If it only lasts 4 seconds, you're doing only a fraction of the damage you used to do
Charis did a little report on this in D2X a little while back and things looked OK - with Totems and Charms it's probably likely to get similar amounts of damage delivered, but only a wait of 4 seconds

May the wind pick up your heels and your sword strike true.