The effect
Following a significant earnings surprise, prices drift in the direction of the surprise over subsequent weeks rather than repricing instantly. The market underreacts, and the correction is gradual.
Why it has not been arbitraged away
Capturing it requires holding many small positions for weeks, which ties up capital and produces returns too diffuse for a fund needing concentrated performance. It survives because it is tedious rather than because it is hidden — which is exactly the profile of an edge suited to automation.
What decayed
The effect is materially smaller than in the original studies and concentrates in smaller, less-covered names where costs are higher. Anyone modelling it on published figures from decades ago will overstate what is currently available.
Effort profile
Once the universe, the surprise metric and the rebalance schedule are defined, this runs with very little supervision. It is a strong fit for automation precisely because its difficulty is administrative rather than analytical.