The modifier block is the block the skull is sitting on top of. Placing a sign on the side of the block the skull is facing can modify it's behavior. Signs are stackable, and you can also put signs on the back side for some modes.
Placing my skull on a bamboo block will display the text on the sign.
Typing HELP into the back sign will give you the display options layout.
Placing my skull on an oak log, and typing HELP into the sign will give you the sign data layout
Placing my skull on a dead brain coral block will interpret the sign data as brainfuck and display the output. The back signs are used for the input.
On top of a loom and with a piano 3 blocks above, it will play a song using the sign data as follows: B1;B2;B3,B4