Question about the speed of the relativistic jets...

[SRH]

I was watching Roger Blandford's SETI Talks and he mentioned in passing that relativistic jets appear to emit radiation at speeds greater than the speed of light, but that this is only an illusion.
http://www.youtube.com/watch?v=flDQEQ5b1pk

My questions are:
1) What was the (incorrect) speed that was calculated by observation?
2) What causes the illusion? I imagine it has something to do with the warping of spacetime, but I have no idea.

Thank you.

[slang]

This Keel guy says:

Strong evidence for relativistic bulk motions comes from superluminal sources, in which the projected speed of motion (always outward from the core) of distinct blobs is 1-10c. A natural explanation is (backwards) time dilation in material approaching us at ~0.9c; the Doppler boosting of this material would make these objects bright, so that the boosted sources were observed first. The emitter stays only slightly behind its earlier radiated wavefronts, so the projected motion is quite rapid (see Superluminal Radio Sources, ed. Zensus and Pearson, Cambridge 1987).

source, with lots more there.

[SRH]

Thanks!
Very interesting.

I've never heard of backwards time dilation before. And nothing is coming up on Google.
If anyone has a link, please post it. Thanks again.