$6 \cdot 90$ Let the numbers $x_{1}, \cdots, x_{1991}$ satisfy the condition
$$\left|x_{1}-x_{2}\right|+\cdots+\left|x_{1990}-x_{1991}\right|=1991,$$

and $y_{k}=\frac{1}{k}\left(x_{1}+\cdots+x_{k}\right), k=1, \cdots, 1991$. Try to find the maximum value that the following expression can achieve.
$$\left|y_{1}-y_{2}\right|+\cdots+\left|y_{1990}-y_{1991}\right|$$