Having identified it, I can't see it's true purpose.  The stock S40 is EU A2 compliant; that is it does NOT produce more than 35KW, and it does NOT have a power to weight ratio >/= 0.2 KW per KG.  It is also compliant for the initial 2 year restriction for the EU A license: that is power is </= 25 KW and the power to weight ratio is </= 0.16 KW per KG.  Stock we produce approx. 23 KW and roughly 0.134 KW per KG based on the stock weight of 380 lbs.  The only other restrictions I can find are for an A3 license, which restricts you to 125cc's or less.

So we know WHAT it is...  but WHY is it there?  

I will try to disconnect it during this or next week, and try to make power test.

How do you suggest me to do so? Accelerating from 0 mph , or when I am riding at mid then give bike full throttle?

I noticed a good power when accelerating from mid to full so I'm afraid of not being able to see any change.

Regarding the gasoline, I don't know what properties does it have here, we have Super 98 octane, and Super 95 octane, being this last one the most used (also cheaper). 98 has more explosion power (as it's said).

The Octane ratings are not probably easy to relate to the US numbers.  The United States and Canada use some averaging thingy where they list our octane as an average between the "RON" (Research Octane Number) and "MON" (Motor Octane Numbers).  Because of the 8 to 12 octane number difference between RON and MON, the  numbers shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel.  Most other countries list an octane rating based on an AKI (Anti Knock Index).  Our common octanes are 87, 89 and 92.

Disconnecting the switch is only a valid comparison if we know that the switch actually works in the first place.  We don't know if the previous owner  did anything to disable the switch like what was suggested on that dirt bike forum.

If you have a multimeter, check the switch to see if it works and at what throttle position it actuates. Simply disconnect it and check continuity until the meter reads it connected.  After that, do back to back rides with the switch connected and disconnected.  If you don't notice a difference...  

I started cleaning my carb today for the first time on my recently-ish aqquired savage and noticed this weird thing. This thread is the best source of information I've gotten to so far but I still lack the understanding of why it sits there. I have a '98 model that's imported to Finland from Sweden so now I know it's a EU thing. The fuel quality theory doesn't seem logical to me since it differs quite much even inside the EU. It goes to the ignitor according to the wiring diagram so the spark timing retardation seems to fit. But why..?

Would be nice to do some A/B test riding but I'm still months away from riding season so chances are I will just rip the thing off and hope the bike still runs... Did anyone ever find anything else about this thing? I know it's an old thread but some of you seem still active!

One of the members who lives in a country where the switch is installed needs to go to a Suzuki dealer and see if they can get a copy (or look at) a factory workshop manual with the wiring diagram.......and see what this switch connects to.  Even looking at a factory parts manual and seeing what they call that switch might help to identify what it does.

When you say it "comes on" at WOT, what is "coming on"?  Does it make a noise?  Flash a blinky light?  Cut off the fuel supply and make the bike starve for fuel because you're hot-rodding it?

I saw the parts diagram calls it a "switch, throttle" (pretty vague and could be a misnomer).  Could it be a thermo-switch heater plug?  My Honda Shadow with dual carbs has a pair of heater plugs.  Expensive little gizmos at $50/pop (basically resistors that look like mini spark plugs).  They're totally useless to me here in Houston TX.  Supposedly they provide a little heat when the ambient temperature gets down to around 32F/0C.  I know some people ride in freezing weather, but not me.