The essential special characteristic of the Chevy 383 Stroker engine is that the crank is 0.270 inches longer. Chevy never made a 383 cubic inch engine. The 383 Stroker is a small block Chevy 350 that has had the crank swapped out for that of a small block Chevy 400.
In the 1970s, a mechanic figured out that if you machined the journals correctly on the crank of a small block 400 cubic inch engine it would just fit into a small block 350 cubic inch engine that had been overbored by 0.030 of an inch. The fact that the crank was 0.270 inches longer increased the rod swing and the stroke length from 3.48 inches to 3.75 inches.
This increased stroke length meant that the stroke was also moving faster if the crank was turning at the same number of revolutions per minute. That means the cylinders are taking in more air and more fuel with increased compression which generates more power and increases displacement. The increased stroke length added 28 cubic inches to the engine displacement, and when the overbore was factored in that brought the actual displacement to 382.6 cubic inches which is rounded up to 383.
The increased piston speed allows the motor run more efficiently at lower rpm, which lets it come up to speed faster.