iMac11,3 and TDM, which cable?

Hi,

I am trying to connect a recent macbook pro (running OSX 10.9.5) that has Thunderbolt ports to a mid-2010 iMac (running OSX10.10.2) that has a miniDP port. The idea is to use the iMac as an external display for the MBP.

I bought a TB cable on the apple website. I thought it would be that easy...

When I plug in the TB cable, nothing happens. OK, so I looked a bit online and found this:

Target Display Mode: Frequently Asked Questions (FAQ) - Apple Support

I tried pressing cmd+F2 when the 2 macs are connected via the TB cable: nothing happens.

Now the aforementioned FAQ page says that I should use a miniDP cable (not a TB cable). Now I assume that a TB cable would be backward compatible, but am I wrong here?

For those familiar with TDM, what exactly is supposed to happen once the 2 macs are connected? Does the iMac should appear as an available screen on the MBP's display options? Should the screen of the iMac automatically turn off and start to display the MBP's output?

Thank you very much in advance for your help.

Best,

-J-