I'm guessing you can high speed sync between Canon and their flashes? Sync speeds are normally 1/180 and 1/200 with normal speedlights, and no where near fast enough to kill ambient. Plus, the location of the flash would have to bear within inches of the subject.
If you have an HSS capable flash, it will sync above X-sync when it's in the camera hot shoe.
If you have have triggers that support a variation on HSS, then the flash will sync above X-sync when used off camera.