Symmetrical modeling can save a lot of time when working on most objects, be it humans or cars. After much digging, I would like to share how this is done.

Modeling certain parts, such as arms, is fairly easy. You simply make your arm, then you edit->duplicate/instantiate->Duplicate Symmetry. This will mirror the arm onto the other side of the specified plane, the default being Y-Z. This is great for pieces that will not be connected with each other, but for pieces like the chest/torso of a human, it would be helpful to work on both pieces at once: making changes to one side and having them reflected in real time on the other.

This is the real key in working with symmetrical models. Start with your primitive, the shape from which the rest of the torso will take shape. Make sure the primitive is to the left or right of the Y-Z plane. Then alt-rightClick the object and go to "symmetrize polygons." This will mirror the selected polygon across the Y-Z plane. You now have the object mirrored as a single object, but we aren't quite done yet.

In order to make the changes made to one part of the object reflect on the other, you need to create something called a "symmetry map." With your new double-object selected, look to the left hand of the screen under "get". Click property, then "symmetry map." This will pop up a dialog box with a bunch of options (that you're free to explore on your own). Now close the dialog box and look to the right of the screen, under the "transform" header. There is a button that says "sym"; make sure this is depressed. Now any selection or transformation done to one side of our symmetrized object will be reflected on its corresponding part on the other side of the Y-Z plane.

Work away on your mesh till you are pretty much done with symmetrical stuff. Now that we have a symmetrical mesh of the torso (or whatever other symmetrical object you happen to modeling), we may wish to add some non-symmetrical elements. In order to return the mesh to a non-symmetrical editing environment, you simply click the "sym" button on the right hand side of the screen again. This causes XSI to ignore symmetry maps.

The final step is to weld to two halves together, which is a simple matter of selecting all of the points on one side that will connect to the other side (this also selects all the points on the other side that will connect to this side), press M, then hold alt and drag the points onto each other. This will weld them together, forming a nice, single, solid mesh. Make sure that the "sym" button is not depressed for this step (you should have turned off symmetry in the last step, anyway), as XSI gets kinda weird when snapping points that move symmetrically. The point moves X units over to snap to the other point, only to have that point move X units in the other direction. It makes a nice vibrating effect, but it's not very useful.

In conclusion, XSI offers modelers the ability to create half an object while XSI automatically creates the other half. Given the amount of objects in our world that are symmetrical, this is a tremendous advantage, not to mention necessary for some objects, like faces. Though the process is not intuitive, once you know how to get it started, it should be a fairly easy technique and a frequent addition to your modeling skill set.