DS cards come in 8MB 16MB 32MB 65MB and 128MB. When a game is written to one, it has "Filler" data to take up the space between the games actual size and ds cart. Trimming the ROM removes the "filler".
The "Filler" is basically non-game data to use the space.
The only negative it has is that it breaks download play, other than that, nothing.
There is, however, a trimmer that detects wifi enabled games and won't break them. It also supports zips and batch processing.
http://techsuki.net/Nintendo-ds-ROM-trimmer/
Credits go to users nintenut and dstrimmer on AfterDawn Forums and to the following thread http://forums.afterdawn.com/thread_view.cfm/505330
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You can't.
Download a SAFE ROM then a Rom reader.
hose hoe hedge trimmer
Redownload the ROM, as it may be a bad one.
If "Rom" You Mean Region It's Hoenn Region I Think? I Helped :)