Iffen you study the three vents, you will notice that one (the one at the neck) is about the same as on other horns. No surprises there.
The other two are part of "one vent", as mentioned above. However, the purpose of the double hole system is to allow the horn to start to speak with the smaller vent, and then only open the whole vent area once the "harmonic has been picked".
It does this by opening the lower vent (with the key action), and then opening the upper vent with a small leaf spring attached to the second key cup. The action of this spring allows for just enough delay to make it all work.
How do I know. Well, I once (pre-New Year's Eve gig) clean off the condensate splash from my horn, and suddenly found out that something was wrong with the octave key. Looking it over, I noticed that the cleaning brush had disturbed the leaf spring and popped it off of its seat. Once it was rotated back into place (where it was held by a small lip on the edge of the bearing surface) all was again well.
Since learning all of this, I find myself almost as distracted by the wiggling of the Yamaha mechanism as I was by the rocker arm portion of the octave key system on my old Selmer baritone. I've learned over the years to get my amusement where I can find it...