From what I have deduced over the past couple years, it works generally the opposite of what the thread title claims. When Bitcoin goes u the big alts go up as well, and when Bitcoin goes down the main alts go down. I think this is because most people think if Bitcoin fails then all ALT coins will fail too, as confidence from the general public about cryptocurrencies would be shattered.
I noticed there is often a time delay between bitcoin going up and the alts going up. I suspect people need some time to get confident bitcoin is really going up before they start investing in alts.
So true!
It takes some hours up to one day to see an impact on alts. Same procedure when bitcoin is going down.
Theoretical it is like an early-warning system and give alt-investors the possibility to avoid bigger losses.
As I said it is all theoretical.