When you try and draw a straight line and it has a bend in it (or a few waves), the next line you draw try and draw the opposite or a mirror image of it.

For example its exactly like how sound canceling headphones work by creating an opposite sound wave therefore cancelling out sound and in our case bent lines.

heres a picture that i think gets the pint across well.

https://mk0soundguyshosprmrt.kinstacdn.com/wp-content/uploads/2017/05/Sine-WavesUCONN.webp

edit: credit to Peter Han who told i learnt this from (i made the headphone analogy tho)