Dr. Geep has an excellent tutorial here in Rendo explaining the theory and use of magnets with models and outfits. 

There are two ways to make body parts or prop/outfit parts invisible.  Go to Window on the taskbar and scroll down the menu to Hierarchy Editor.  That will open the Hierarchy window and you will see the hierarchy chain of props and characters listed.  By each part, there is an eye icon which indicates that each part is visible, can be seen in your scene.  Click on the eye icon to render that part invisible.

The other way, go to Object and on the menu click Properties.  The window will open.  Make sure you have your model and body part selected in the Preview window.  Under the Properties tab you will see several labels and tick boxes.  Uncheck Visible, Visible in Raytracing and Casts shadows.  And that's it.  Your body part is completely invisible and will not add any additional time to your render process.