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I've always only used the built-in optical signaling. Optical signaling has gotten a bad reputation because a lot of people just don't understand how to use it effectively. That's not the say radio triggers aren't better, just that you shouldn't automatically assume you need them, buy them, and never learn to use the Canon system.
When I bought my Canon flash I'd been using pairs of optically triggered manual Vivitars for 30+ years so I was well aware of the limitations of optical triggering and manual power setting. I bought Canon because I wanted the convenience of ETTL in action situations and HSS outdoors. The Canon system works well indoors because the slave doesn't need line of sight indoors it just needs to see enough light from the Master directly or bouncing off the ceiling. Outdoors line of sight is needed between Master and Slave because there's nothing to indirectly bounce the light towards the slave sensor.
While there are some situations where you might want to trigger a flash from 50 or 100 feet away that's not the case for 99% of the flash shots I take. Again you need to remember its the downward angle of the light which makes light naturally flattering and as the distance increases beyond about 15ft the angle decreases unless you raise the lights higher on tall stands.
The slave sensor is the small gray lens on the front of the base. Common sense will tell you it needs to be aimed wherever the light from the Master is strongest and not blocked by a modifier for the optical signaling. Those two criteria were taken into consideration when I created my diffusers. They sit on top of the upright flash which makes it easy to orient the base towards the master on the camera, and then turn the diffuser to the subject.
There's also a lot of misunderstanding about the cause and effect of creating "soft" lighting. "Hard" and "Soft" are perceptual reactions to the highlight placement on the face and the shadow tone clues. Flash on camera looks "fake" in part because the highlights the flash creates on the cheekbones are lower than those seen in natural downward light. Raising the flash on the bracket doesn't make it any "softer" but it moves the highlights up to the top of the cheeks, chin and lips where they look more "normal" and natural.
The mood and character of lighting is controlled with the tone of the shadows and that's a function of the fill intensity relative to key light. When using single flash you need to rely on "spill fill" off the ceiling. Even when using direct flash or small modifiers like mine there is a lot of spill fill generated. But with one flash you can't control it the same way as with a two flashes with separate centered fill..
With a separate centered fill source, like a Master flash on a bracket over the camera, you will get smooth open shadows with front > back fall off. Many assume shadow transitions are entirely the result of "wrapping" a large key light around a face when in fact what the larger source tends to do is bounce more spill fill around the room and into the shadow side opposite the key light. With centered shadowless fill you don't really need a huge key light modifer, as illustrated in this comparison...
The kids were visitors who had dropped by after a day of sightseeing. I grabbed the camera with speedlights to shoo the boy then finding they had time switched to my studio lights with much larger modifiers. The interesting thing about that comparison is the fact that light on the girl seems harsher than the boy despite the larger modifers and lighter shadow tone. That's because the boy had washed his face when he arrived removing the oil but his sister wearing make-up didn't. Shiny skin trumps larger modifiers in the highlights, but the overall mood of the lighting and transitions on the shadow side are controlled mainly by the front > back fall off of the even centered fill.