TeX interprets 'true' as a keyword when it occurs before one of the fixed units pt, pc, in, bp, cm, mm, dd, cc, or sp. It signals TeX to rescale the corresponding dimension so that any magnification of the document will result in the dimension initially given: The total effect is that magnification will not occur on this dimension. For example, recall that \hsize is a parameter that holds the width of a line. The following will enlarge everything on a page by a factor of two but keep the line width fixed at 5in.
\magnification = 2000 \hsize = 5 true in