First one is the custom selection.. this way you can make moon shape selections triangles what every you want to..
Second is deleting not 100% but 50% or what every opacity you want to delete from the object you working on.
Ok let start
1)Open up a new document 300 by 300 pixels.
Create a nice big rectangle in the middle give it what ever colour you want to, made mine blue
2)Give it this bevel and emboss settingsyou will see later why.
3)Now make your selection with the Elliptical Marguee Tool around the size shown in the picture below
4)Now hold down ALT and do another selection which will result in cutting down the first selection until you have the shape shown below.
*tip* if you use schift instead it will merge the new made selection which will result in a bigger selection field.
5)Now pick your eraser tool from the left and use these settings ( notice the opacity put down to 50% )
6)Now go over the selection while hold down your left mouse button
7)Well because we want the precise same selection below the already erased selection use your arrows to move it down or once more select selection elliptical marguee tool and drag it down with your mouse
8)Repeat step 6 and you will have this as finished result
I know it's not that fancy but it will come in handy for alot of things
extra: playing around with the opacity settings will result in some fancy effects