They simply load somewhere on the DNA and start shaping it into a loop until they reach a point where they are forced to stop. That's why they rely heavily on the ability to explore both sides of ...

"We were surprised to find that the same core principle of sequential and hierarchical DNA loop formation is used to either tightly pack chromosomes during division into safely movable entities ...

The paper M. Ganji et al., “Real-time imaging of DNA loop extrusion by condensin,” Science, doi:10.1126/science.aar7831, 2018. How does a human cell neatly pack ...