>
>
> What is the sense of \startPSTRICKS ... \stopPSTRICKS ?
>

Maybe he meant:

\startpspicture

\stoppspicture

The actual macros of pstricks are defined in pstricks.tex and that is all
Plain TeX. I myself generally would do something like this in Plain TeX

\input pstricks
\pspicture(0,0)(2,2)
\psline(1,1)(2,2)
\endpspicture

\bye

and In LaTeX actually if you want to define an environment, you can use
\def\environment and \def\endenvironment. I guess then you could probably
use it in ConTeXt as

\startpspicture

\stoppspicture

and you can probable load pstricks by saying \usemodule[pstricks] or
anything that context uses.