hi there folks.
my question.
i know how a unlock works,but .........how cann i lock the phone to a specific carrier (example Orange)

Welcome to the forums.

If you enter the unlock code incorrectly enough times (I believe 5) it will lock the device to the carrier the device is branded for.

If you're talking about the network you want to use, with no roaming, you would go to Options > Manage Connections > Mobile Network Options and set Network Selection Mode to Manual. You should be prompted to choose a carrier from a list.

I need to unlock my Blackberry from a different network if i do so will it affect my BB pin? and will i still be able to use it that same as before?

You can't change the pin...

You need the subsidy unlock code. That way you can use a Vodafone BlackBerry on O2, for example. Horizon Wireless can get it for you, for a fee.

If your PIN is associated with the old user's data plan and you cannot set it up with your current carrier (even though it's unlocked), you have to get the previous user to call his carrier and get it released.

thanks friends!
but i must be writing my question wrong )

for example : supposing i bought a unlocked BB from ebay, now when i want to connect it back to one of my carrier's BIS,they just conect it AND THEY JUST KIND OF LOCK IT BACK )) i really dont know how they're doing that ))

Wrong.
Unlocked means use with any carrier's SIM.
The BIS is linked to your phone's PIN, so you can get the service books, and set up email etc.
BIS doesn't LOCK the phone.
You can still use it with any carriers SIM, however the BIS site is setup for ONE carrier.
TO use a different carrier, you need the blackberry data plan on the other carriers SIM; you need to call the previous carrier and ask that your BIS account be deleted. Then you create a NEW BIS account on the NEW carriers page.

You don't. Why would you want to? That is, unless you're confusing the subsidy lock with the BIS association, as Midnightraven pointed out.

Once a BlackBerry is unlocked, you CANNOT re-lock it even if you want to. Updating the OS won't do it, nor will installing any piece of software. So once it's unlocked you can put in the SIM card for any carrier and it will work. You will need a BlackBerry data plan from whatever carrier you use if you want all the BlackBerry services. But the phone and SMS will work with anybody.

Thanks a lot!
that is what i wanted to hear "Once a BlackBerry is unlocked, you CANNOT re-lock"