A footprint condition at price P_i where bid volume exceeds the ask volume one tick higher by a defined ratio: fracV_bid,iV_ask,i+1 ≥ R_threshold. Common thresholds are 250% (moderate), 300% (strong), and 400% (extreme). A selling imbalance flags concentrated aggressive selling at that level — the diagonal comparison shows sellers hitting the bid far harder than buyers were lifting the offer just above.