how does work S-Function block in simulink?

Hi dears, I know that S-Function block is an outdated block in simulink. but I need to understand how it works because I have to decode an old file. The used code in this s-function is like below. If I enter constant value, a curve is seen,that differences from zero up to the stated value. I dont know what happen on entrance data. any idea?

Thanks a lot, Javad

function [sys,x0] =FT_Ball(t,x,u,flag)

kb=12;

switch flag

case 1

    sys(1)=x(2);
    sys(2)=x(3);
    sys(3)=kb^3*u - 5*kb*x(3) - 6*kb^2*x(2)-kb^3*x(1);

case 3

    sys(1)=x(1);

case 0

    sys=[3,0,1,1,0,0];

    x0=[0,0,0];

case {2,4,9}

    sys=[];

end

Answer 1

[TAB]

See Level-1 MATLAB S-Functions.

[Ted]

Thanks ... but I cant understand explains clearly. Can you explain the notes according to my code ,please.