The 'calling' of the true name is a little different from chanting. The heroic spirits call out the true power of the weapons by calling out their names, thats what makes them 'owners' while Shirou/Archer/Gilly are 'wielders' as per the routes words.
The chant seems to work more like a Magi's chant to invoke a change in reality and bring the mind-world of the reality marble out and make it physical...thus the self-hypnotism...
While we're on the subject of UBW
Does anyone have the version of this image without the poem and with Saber? I cant seem to find it... no matter how hard i look >_<