Example of ineffective poaching. You are in an area that the thrower cannot or will have a hard time throwing to. You are not helping your team and are not in a position to make a defensive play on the throw.
Example of effective poaching. Although the person you are supposed to be defending is open, they are on the break side of the mark. In this position you are cutting off available upline throws that could otherwise be open. I would not recommend placing yourself here unless the thrower is weak at breaking or there are immediate upline threats.
Effective man to man defense. This is the safest of them all as your person is not open. Assuming everyone else is playing tight to their man this is the best possible position. Especially if the thrower is good at recognizing poaches and/or breaking the mark. Prioritize preventing an upline cut over a backfield dump.