Question about Y-Splitter Cables

[Bleedman0xx]

So, I was browsing through eBay, looking for Y-Splitter cables for my 3-pin case fans so that I can use them with my fan controller (Lamptron CW611) that I recently purchased, and I found a listing that i liked.

 

http://www.ebay.com/itm/3-Pin-Computer-Case-Fan-Y-Splitter-PC-Power-Adapter-Cable-w-Black-Sleeve-6-/121137384802

 

In the description it says:

Note: Only one 3-Pin male connector has a tachometer/speed wire. This is because motherboards/fan controllers cannot accurately read the fan speed when two speed signals are being given simultaneously. This cable is made correctly to avoid this problem, with only one tachometer wire on one of the male connectors.

 

My question is; Can somebody explain what the tachometer wire actually is? I have a feeling I already know what it does, I just want to make sure. Also, if I buy these cables in the listing above will I be able to customize the RPM for both fans to run at the same RPM when connected to this Y-Splitter cable as I normally would with my fan controller?

[Godlygamer23]

It's all about fan speeds. With the one connector, you'll be able to monitor the speeds - or at least you should.

[Bleedman0xx]

It's all about fan speeds. With the one connector, you'll be able to monitor the speeds - or at least you should.

So it is as I thought then? It's just a wire that translates the RPM from the source and isn't actually responsible for manipulating the RPM?

[Phalonx]

Yes.  It will read the RPM only.  This is why there is only one such wire between the two - otherwise you get two readings on one input.  In order to control the fan speed digitally you'll need a four wire fan - the fourth wire would be PWM and is used to control the speed.