Impact Stamp slams your word down onto the surface hard enough to shake the frame, with the letter edges roughening on impact like ink pressed unevenly into paper.
st-slam drops the word in with an overshoot and a brief frame shake, while feTurbulence and feDisplacementMap roughen the glyph edges — a stamp that landed with perfectly clean edges would look printed rather than struck.
The displacement map roughens them on contact, imitating ink spreading unevenly under pressure. It is strongest at the moment of impact.