I have a confusion about the following proof
Here it says the the transition function from $U_1$ to $U_0$ is multiplication by $x_{0/1}$. I don't understand what that means. My understanding the transition function should be a map from $\operatorname{Spec}(k[x_{0/1},1/x_{0/1}])$ to $\operatorname{Spec}(k[x_{1/0},1/x_{1/0}])$. I don't understand how is multipliction by$x_{0/1}$ is a map from $\operatorname{Spec}(k[x_{0/1},1/x_{0/1}])$ to $\operatorname{Spec}(k[x_{1/0},1/x_{1/0}])$.