The moment it goes down
Color goes into a stamped slab first, either batched into the ready mix or broadcast onto the floated surface as a hardener. Release agent comes right after that and right before the mats do. Without it, a rigid stamp mat pressed into soft concrete would pull the surface up with it when it lifts back off, tearing the pattern it just made. A dusting of release agent between the mat and the concrete lets the mat seat to full depth and lift away clean, which is what lets a crew move a stamp mat across a whole slab in sequence instead of fighting the surface at every step.
Not the same job as a sealer
People hear that stamped concrete needs a release agent and a sealer and assume they are the same product doing the same job twice. They are not. Release agent goes on while the concrete is still soft, during placement, and its job is finished the moment the mats come up. Sealer goes on weeks later, after the slab has cured, and its job is ongoing, protecting the color and the surface from traffic, weather and stains for as long as the slab exists. One is a placement tool used once, on pour day, and gone within hours. The other is maintenance you keep up with for as long as you own the slab.
What survives the wash down
Most of a powder release washes off once the slab is pressure washed after it firms up, but not all of it. What stays behind settles into the low points of the pattern, the grout lines and the deeper texture, leaving a darker tone there than on the high points. That contrast is a big part of what makes a stamped slab read as real stone or slate instead of a flat pattern pressed into gray concrete.