Hardpack over refreeze is the hardest surface imaginable to ride on. The ruts in the refreeze try to make your wheel follow them, and the hardpack on top shifts out from under the wheel without warning.

It takes patience and finesse to manage this surface. If you try to brute force it, it doesn't work quite as well.

When I encounter it, first I slow way down, shifting to my granny. Spinning like crazy helps maintain, and regain balance. Then I shift my weight back on the bike, so the front is as unweighted as possible. This lets it shift side-to-side without throwing me around. Finally, I keep my grip and upper body loose to go with the flow.

This seems to work remarkably well. The hardest part is that it runs counter to my first and natural reaction, which is to strongarm it and tough it out. That never ends well for me.