When the file is stored normally, you look up where it's at in a directory which is organized logically. So if you want the file "Abc123.txt" and you know it is stored at "C:\MyFiles", you browse to "C:\MyFiles" and pull it up.

Behind the scenes, there is additional information about where that file is physically stored on the disk. Maybe it's on disk 1, platter 2, track 15, sector 47. You don't really need to know all that information, but the system does. The system also knows that it can't use that particular location for new files. It's reserved.

When you move the file to trash, the directory is updated so that "Abc123.txt" now shows as being in "System\Trash". The file is still physically located in the same spot on disk 1, platter 2, track 15, sector 47, and it is still a reserved spot. It just doesn't show up in the regular directory.

When you remove it from trash, the file is still in the same physical location, but that spot is no longer reserved. When new files are added, the system can overwrite that physical location with new data, but if no new files are added, that physical location will still have the data. You just can't access it because you can't tell the computer directly to get the data from disk 1, platter 2, track 15, sector 47. Well, you can with specialized tools, but not from your regular operating system.