6. For real numbers $u, v$, define an operation “ * ” as: $u * v = uv + v$. If the equation $x * (a * x) = -\frac{1}{4}$ has two distinct real roots, then the range of real numbers $a$ that satisfy the condition is . $\qquad$
II. 6. $\left\{\begin{array}{l}
a>0 \text{ or } a<0 \\
a^2 - 1 > 0
\end{array}\right.$
Solving, we get $a>0$ or $a<-1$.