Whoops, common mistake at the top of the article. Explain the ST-E2 then.
Canon light triggering is NOT an infrared system.
It uses visible light pretriggering.
The red panel on your flash is for auto-focus assist.
Canon does list this function as an "optical" triggering system, though I'm curious myself how the ST-E2 (or the ST-E3-RT for that matter) work in this regard... or maybe I'm not that curious because we're living in a radio triggered world now!
Ok, here's the deal. The Canon 'optical' triggering system does use infrared light for the triggering, but depending on the source of the triggering signal, visible light may also
When using a flash as a master, the signal is sent by the main flash tube. The xenon flashtube in a Speedlite emits a broad, relatively even spectrum in the visible range (useful for a 'white' flash with an approximate daylight color temperature), but there is even stronger emission in the IR range. The emission spectrum looks like this:
The receiver on a slave flash is covered by a filter 'window' which blocks the visible light, but passes the IR light - those big peaks between 800-1000nm. If you look on the front of a slave-capable Speedlite, there's a black panel which covers the IR receiver, and on flashes with an AF assist lamp, there's also a red 'window' for that (the slave receiver is usually above the red AF assist lamp panel).
The ST-E2 master has a xenon flashtube inside it (the AF assist lamp is separate), and that's behind a 'black' filter (the main 'panel' above the AF assist lamp window), which blocks the visible portion of the xenon flashtube emission, so only the IR component of the flash is emitted to control the slave units.
I suppose Canon calls it 'optical' triggering because most of the time (except with the ST-E2), there is visible light emitted along with the IR triggering signal. But it's also correct to refer to it as an IR triggering system, since that part of the spectrum is what the receiver is actually detecting.
Or, if you prefer, Canon light triggering IS an infrared system
The ST-E3 emits only RF - no IR (optical) triggering, and no AF assist lamp, either.