Can a shape key be negative? It seems shape keys can't manually be set to a negative number or a number larger than 1. I've even tried using a driver to drive the value of the shape key beyond either of these extremes with no luck.

What I am trying to accomplish is a single shape key of an object bending forward, with the hopes that a -1 value of the shape key will bend it backwards as well, so that I can drive it with a sin function so that the bend oscillates back and forth without having to animate and synchronize two separate shape keys.

Is there a way to push a shape key into the negative? (And as a secondary question, can it be pushed past 1?
You can set the Min and Max Values in the Range fields: