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And if you are chasing a Railgun/shaft and they fall off on purpose you will get mad or frustrated that your kill is wasted and so this is why tanki did this (READ FULL DETAILS ABOVE)

:)
After self-destructing you lose a point because there are a lot of players that 
self destruct themselves because of low health and they want to respawn back to full health.
This is Very common in Tanki Online, But even though you fall down accidently You wont lose Xp
You will only lose the score in the statistics of the match (if you dot get it read on)

Example Your score is:::: 556 you killed like 50 people and you died 45 times
When you self destruct the scores reduction is depended on how much health you had before
Self-Destructing.If your health was all the way at the very end and you self-destruct your score will reduce more but if you self-destruct when your health is at the start you wil only lose 4-6 Battle score NOT the Xp above. 

So the reason Tanki did this is to keep players away from self-destructing just to re -spawn (in other word get back to full health) So whenever you self-destruct to respawn you will lose your battle Xp NOT your XP above the page.

Hope this helped :-)
And.. If you wanted to know when the gold box drops,
The gold box drops when a surtain match is getting very intense
Both teams are equal and everyone is in a race to be 1st in the stats list
(Itmight happen randomly too)I hope i answered your question
Hello Eyal If you want to know where the gold box drops
For "Some" maps not "all" maps here is the link:
http://en.tankiforum.com/index.php?showtopic=234421&page=1