Individual clues can be weak while the line as a whole is strong. Joining forces means reading the clue list as a single unit: compute its minimum span — the sum of all clue lengths plus one mandatory gap between each neighbouring pair — and subtract that from the line length. The result is the line's total slack: the number of spare cells the whole caravan of runs can shuffle across.
Slack is the single most informative number about a line. If it is 0, the line is fully determined: write it in. If it is small, every run is nearly pinned — each run of length k yields k − slack certain cells. If the slack is larger than every clue, the line gives you nothing yet; spend your attention elsewhere. Clues 3 4 in a width of 10 have a minimum span of 3 + 1 + 4 = 8, so the slack is 2, and both runs leak certainty:
The second half of the technique is directional: a clue list reads left to right, but you may solve it from both ends inward. The first clue is pinned by the left wall, the last by the right wall, and each cell you resolve on one end reduces the effective line length — and therefore the slack — for everything that remains. After any progress at the edges, recount. A line that started with slack 5 might be down to slack 1 in the segment that is still open, at which point the overlap method suddenly fires inside it.
This is the quiet skill separating fast solvers from careful ones: they are not staring harder, they are recounting sooner. Slack arithmetic costs seconds, works on any line without touching the board, and tells you precisely which lines deserve a closer look with simple boxes or edge logic. On a 25x25 daily, a slack census of all fifty lines is the fastest possible orientation pass.