Have you been playing with the Joint Editor? That's the sort of effect you get if you move the fall-off zone spheres (especially the inner fall-off sphere). Or, if you haven't moved the fall-off zone, maybe you've shifted the arm/hand with the magnets. It's important to remember that if you change any part of the mesh with magnets, wave deformers, or applied morph targets, the rigging (bones) does not move with the deformed mesh. When you start to apply poses, if the centre point and fall-off zones are now in a different place relative to that part of the mesh, you'll get poor bending (or breaks in the mesh).

Example .. imagine you create a morph for your model in an external 3D prog. Let's say the morph elongates the figure's body from the stomach upwards by 50%. You apply the morph to the model in Poser, and all looks well. But, if you opened the Joint Editor (or looked at the bones in the SetUp room), you'd notice all the bones are exactly where they were in 3D space for the unmorphed character. So where the arm bones meet to form the elbow joint, is now about where your elongated model's wrist is. Apply a pose and all the bones are affecting bits of mesh that are now miles away from where they should be. Ditto the fall-off zones.

Post a screen-shot of your model with the hand selected in the Joint Editor view. All will become clear.