As I understand it trackers are linked to the ECB rate so if there was a change in the ECB rate the rate for tracker customers automatically reflected the full ECB rate change Eg if you had a margin of 1.1 above the ECB rate and the ECB rate increased from 2 % to 2.25 % the tracker customer rate would change from 3.1 % to 3.35 %.
As you rightly say variable rates follow the ECB rates including SVRs but generally at the bank discretion. They may or may not pass on an ECB rate change to SVR customers and may not pass it on in full .
With respect to KBC cases, for those who started on discounted rates to roll onto prevailing rates as opposed to a fixed rate, if they received letters passing on in full ECB rate increases in 2006, this may evidence that they were on discounted rates linked to a tracker product as opposed to an SVR product. I don't think changes in ECB were passed to SVR customers in full in those years if at all in early 2006.
And back to a constant query on all the KBC threads , did they even have an SVR product 2005-2007 for residential and buy to lets