One idea at a time · 2
A line is not broken alone.
The obvious way to break a paragraph is greedy: fill each line until the next word will not fit, cut, repeat. Every early line is comfortable, and the cost is deferred — the last third pays the bill in stretched spaces. Knuth and Plass (1981) instead minimise the total badness of the paragraph, considered together: one line agrees to be set a little loose so that three lines after it can be set well.
Both paragraphs below are justified to the same measure. The tint behind each line is its tension — how far its spaces are stretched or squeezed from natural. Drag the measure and watch where the strain goes.
Greedy
Knuth–Plass
Red is strain. Greedy piles it at the bottom; Knuth–Plass spends a little everywhere and never much anywhere. The last line of each is set flush left, never justified — a paragraph ends by stopping (MB p.80).