A volume spike is a bar (or session) where traded volume significantly exceeds its recent average, commonly measured as a multiple of a rolling 20-period average volume. On its own it's not a directional signal; it's a confirmation filter layered on top of a price signal.
The logic: a breakout or crossover on thin volume is far more likely to be a "fakeout," a brief poke through a level with no real participation behind it, prone to snapping back, than the same move on volume well above average, which suggests genuine broad participation.
Several of zengtrade's breakout and momentum strategies require a volume spike alongside the price trigger for exactly this reason. It's specifically the filter that rejects thin-volume breaks, which back-testing showed were a major source of false signals when the price trigger was used alone.
Educational content, not investment advice. zengtrade is paper-first and non-custodial.