The slake test (wet aggregate stability) scores how well your soil structure holds together in water. This ranges from 0 being unstable with no aggregates to 8 retaining more than 90% of it’s structure when submerged and shaken in water.
Healthy soil has good organic matter content and is full of microorganisms that secrete glues which create the aggregated crumb structure we want to see.
When aggregates with a good soil structure held together by organic matter and biological glues are submerged in water they stay together, even when shaken about.
When aggregates with a poor soil structure held together by compaction are submerged in water they fall apart, without even being shaken at all.