Recent HGV/cyclist accidents

This may sound stupidly simplistic- but what is "source"?

Is it as simple as if you are not too the left of an HGV then it cannot squash you (whilst turning left)?

That's a pretty simple message to try to get over.

What happens if the HGV screams pasts you and then turns left on top of you.

What happens if the HGV overtakes you then gets halfway past and starts pulling in to the curbs and leaves you no where to go?

Not that simple is it