*rukkjan

The core idea of this root was something like

quick, sudden movement

Think for instance of the phrase “rock the boat” or even just rock music. In English, the verb to rock is the only relevant offspring of the family, but in German, we have two main branches – rücken and regen. And both have shifted somewhat away from the notion of sudden motion toward just “moving”.

The verb rücken is about moving something a bit across a surface and the best visual is to think of you shifting your chair back when you want to get up.

regen is more aloof and captures the general idea of starting to move after being still.
Both are quite useful families that are worth exploring.

Neither the English noun the rock (stone) nor the German der Rücken (the back) belong to this family. They just happen to look like they would.