Answer: the demand for treasury notes.
Explanation: One of the factors in dollar value is the exchange rate,but dollar however moves in synchronization with the demand for treasury notes.
In the US, the department of the treasury sells notes for an interest rate that is fixed as well as the face value.
At a treasury auction, investors now go ahead and bid for less or more than the face value,and from there,they can now resell what they got on a secondary market.
When we talk of high demand,we mean that investors actually pay more than face value and also accept a yield that is lower whereas low demand entails investors paying less than the face value and receiving a higher yield.
But a high yield means a low demand for dollar until the yield goes high enough as to trigger a renewed demand of dollar.