[latexrefman] ‘Hef\-\linebreak feron’

[Karl Berry]

    only when you pass to it optional priority level 0 to 3, w/o optional
    argument the break is forced isn't it ?)

Yes, that's correct. ltspace.dtx says:

% Default argument = 4.  Puts a penalty into the [...]
% output as follows:\\
%                   0 : penalty = 0\\
%                   1 : penalty = |\@lowpenalty|\\
%                   2 : penalty = |\@medpenalty|\\
%                   3 : penalty = |\@highpenalty|\\
%                   4 : penalty = 10000

So \linebreak with no optional arg forces a break. -k