Some of the Canon EOS range have the ability to wirelessly trigger external flashes from their own, on board, pop up flash.
This works by the flash sending out a very quick "burst" of flashes like a code, immediately prior to taking the photograph. The flash unit itself, separate from the camera senses these flashes from the camera body, telling it to fire, and with what settings (flash compensation, 1st/2nd curtain shutter, etc). It can control different groups too, up to 4 I think (?) with effectively an infinite number of flashes in each group.
I use this feature fairly frequently with my EOS 7D, and Speelite 430 EX II. The two work perfectly together. The menu settings for the 7D allow the body to be used only as a trigger for the speedlite, and also as a trigger and then flash as well.
So long as the speedlight is positioned so as to "see" the bursts of light from the on-camera flash then it will fire. (Useful for strange angles, or hiding it round a corner). ie. it doesn't need direct line of sight to the camera, but does to the flashes of light generated by it.
As for whether the light from the external speedlight "overwhelmes" the subject, well that all depends on your setup - but with a bit of thought you can really light your subject well with it and not at all harshly.
Unfortunately, your XS is not equipped with this capability. I believe only the EOS 7D and 60D have this capability.... (I could be wrong but have checked the manuals online for the 1000D, 500D, 550D etc and they do not have it)....