As far as I know the effect of bitcoin price when it goes down is the worry of the majority the word panic. This is the clear effect because profit will decrease also more than half. When it goes up then almost altcoins will become green color. That is how I notice bitcoin price effect.
Depending on the stage, the impact of bitcoin on altcoins will be different. But usually, when bitcoin goes up, immediately the altcoins will go down, then go up again.