Concentration measures how much of an index move its largest contributors accounted for. WMTMT states it as the share of the day's move contributed by the top three names: 30% is a reasonably distributed session; 70% is a narrow one; and readings above 100% are real and revealing, meaning the top names contributed more than the entire move while the rest of the index, net, went the other way.
Those over-100% days are the ones the headline number hides completely. An index that closed up 0.3% on 200% concentration was actually a down market with one enormous exception. The Dow does this vividly: one high-priced stock having a strong day can outweigh twenty members falling.
Concentration is not inherently bad; a giant reporting a genuinely good quarter should move the index. The point is to know. A market where the same handful of names is the whole move, day after day, behaves very differently in a drawdown than one where gains are broadly earned.
The attribution map makes concentration visible at a glance: cells sized by points contributed, so a narrow day looks narrow. The daily notes track it in writing.