Your link doesn't work, but I'll give it a shot. A transaction basically says "I'm authorized to use the outputs of this other transaction because I know this key, and I choose to send it here." So let's say your wallet A has key A for Transaction 1 which has 10 Bitcoins, and you back it up. Now if you authorize 1 Bitcoin to be sent to your same address that key A has, using Transaction 1 as the funding source. The client has to make use of all the funds in a transaction. By default, the client will create a new key, Key B, to send the other 9 Bitcoins too, and those will be stored in your wallet. However, if you delete your wallet and restore the backup, it only knows about Key A, not Key B. Therefore, the 9 Bitcoins in change you sent are gone forever as there is no way to get that key back.
Hope that helps.
I think i get it. It is so dangerous. OK lets say
If this time someone sent me 1 BTC after i backup 5BTC and then load back that wallet. Will i lose 4 BTC as well.