Usually you have 5 changes to change the region of your DVD drive. If you have used them all, the only way to change again is to let the vendor reset the change counter for your drive and reducing the number of changes the manufacturer is allowed to issue by one.
Unfortunately most vendors do not actively support this feature meaning that you can only change the counter 5 times and that's it. Here's where the NEC RPC2 Reset tool can be used. Once you have no region code changes left, the "Reset" button is activated. By pressing this button you are able to issue the command that NEC would use to give you 5 more changes.
Once all 4 vendor resets are used, you would be stuck with the last region you set. But now instead of the "Reset" button, you can now press the "Global Reset" button which modifies some values in your drive in such a way that it behaves as if a region has never been set before.
Of course this sounds great and you think that you could change the region code as often as you want to. Unfortunately the location where these settings are stored in your drive only allow a limited number of changes. Using this tool a couple of times surely does not hurt your drive, but you should not use it too often. If you really need to watch DVDs with different regions, it is highly recommended that you use a modified RPC1 firmware instead.