A good quality Barlow is essentially 'invisible' and does indeed multiply the magnification by the Barlow factor. However the eye relief is extended on a Barlow to a point whereby longer focal length eyepieces become unusable.
A Powermate is a 4 element Barlow and has the advantage in that the eyepiece sees the same light cone for which it was designed and thus the eye relief is not altered and there is no vignetting.
See
http://www.televue.com/engine/TV3b_p...k#.Uzkfq6iSySo for a good explanation on Barlows and Powermates and the differences between.
See also
http://www.televue.com/engine/TV3b_p...k#.UzkhyaiSySo which is specifically on Barlows. (The first link above is a very good read).