Video transcript
Column two needs four connected squares. The run can occupy rows one through four or rows two through five. Those are the only two placements for a run of four inside five cells.
Compare those placements: rows two, three, and four stay filled either way. Those three cells are guaranteed. The top and bottom cells each disappear in one legal placement, so this clue alone cannot determine either end.
This is overlap. Do not choose the arrangement that looks nicer; keep the cells every possible arrangement has in common. If no cell appears in every legal placement, leave the line alone and return after a crossing clue gives new information.
The same reasoning works in column four. Its middle three cells are guaranteed, but the ends remain unknown. Unknown does not mean empty, so do not cross them out prematurely.
The other clues eventually settle the remaining cells. Try the exact puzzle from a blank board, first starting with overlap and then starting with the full lines. Different first deductions can lead to the same logical solution.