If you are playing with your head on the table, other players can’t attempt to physically dislodge you. They can try and talk you into moving.

You don’t have to put your head on the table if you don’t want to. The enters-the-battlefield ability will still destroy the target creature, and Skull Saucer will then be sacrificed.

You can use any part of your head, such as your chin.

If you’re not playing on a table, putting your head on the playing surface will count. (Wait, what are you playing on? Doesn’t matter.)