AHA! TOLD YA SO!

http://www.belloflostsouls.net/2012/04/40k-knowledge-bomb-i-can-ride-bike-with.html

Quote :

Those players often then declare that Ghazghkull is leaving the unit to trudge ahead. Let's say for instance that he moves 4". They then take the boy unit he was with and moves them up '6 and re-attaches Ghaz to them. This is not legal....

Somebody tried to conveniently remember rules that favor them and forget those that doesnt?

@red: this issue was heavily discussed in the old forum, and there wasn't a decisive conclusion about it..

Still not an faq, but the comments are quite convincing. So far I haven't seen one favouring the slingshot maneuver though.

There are legal slingshot if you know how to. But the example given is not a legal one and I care not to elaborate any further.

Of course there are other ways to do it, I'm just saying the comments just shows that more people do not approve of it. The article itself have not given in depth details on that issue, like not even stating why it's not legal.

One particular comment suggests you can only intend to either join or leave the same unit, and that intention carries on for the rest of the movement phase, indicating you can't change that intention at the end of the phase.