Hello!
As the demand for a good increases, the demand for a related good also increases. As the demand for a good decreases, then the demand for a related good decreases as well.
For example, if a new DVD player is released, then consumers will want the new product, therefore increasing demand for it. Because the demand for the player goes up, the demand for DVDs will also go up because consumers will want more DVDs to use with their DVD players. In this case, the DVD is the related good.
On the other hand, the demand for a related good will decrease if the demand for the main good decreases. For example, if demand for potatoes were to decrease, then the demand for potato peelers would also decrease. This is because people would not need potato peelers if they do not purchase potatoes.
I hope this helps you! Have a lovely day!
- Mal
