I have the straight line chalk real, i borrowed it from a friend, but i don't understand how to make the line "snap"?

The string should be stretched tight over two marks to strike a line between. If you can't put the hook over the end of a board or on a nail you will probably need a friend to hold one end. After stretching the string tight between the two marks grab the string with your free hand, lift it up about 8 inches or so and release. The string should snap back to the surface laying down a straight line of chalk between the two marks.