Details of disk defragmentation

Disk defragmentation is used to organise the contents of a disk and speed up the process of opening a file. But recently i heard that when the process of defragmentation is in progress, if the computer is switched off (improper shut down, by directly turning off the processor without going to task bar and using shutdown menu) the contents of the disk will be lost !!!. This makes me afraid and i have not defragmented my drive. is it true. ???