the way I'd do it is to model the depression (create a sphere, delete half/two thirds how ever deep you want the dent) then flip the face normals of the remains. This will be the dent/depression (use the poly sculpt tool or the move components tool with randomize if you want it to be an irregular dent) now move it to the surface of your polygon, cut a hole in the polygon with the polySplit tool that roughly matches the perimeter of the dent, combine the two objects together then weld the new hole verts to the dent perimeter verts..
This is a good method because you can copy the dent to multiple places before you attach it
There is a slightly more professional way to achieve this using fill hole and make hole but it's too complicated to try and explain (not that it's complicated to do, just to explain without some kind of example or images)
a second method would involve using extrude.. select the polygon to recieve a dent, extrude it, just scale it into itself then poly split this polygon's edges and move it's verts until it resembles the dent shape, then keep extruding/scaling it inwards until it forms a nice dent depression.
alternatively, if your polygon has a lot of subdivisions you could just use the polygon Sculpt tool set to push.
:nathaN