That's strange. I upgraded my locks a little while ago, so I had to adjust both mechanisms so they worked with the new locks. I thought the original mechanism allowed you to lock it from the inside, then slam the door & it would lock. With the door ajar, sliding the lock on the inside moves a cam against the flat plate above the mechanism in the door, then when the door closes & that big heavy C-shaped lump in the lock rotates against the door jamb section. The thin plate slides downwards, allowing the cam to slot in & prevent the plate moving upwards again.
Is it an incorrect memory of how it worked, or maybe it only worked with old locks?
I don't have one here to look at ATM, but I have a spare at home, so I'll try to see tonight. Magic smoke escaped from home computer, so if anyone else can jump in a bit quicker....
BTW, you don't need to shed kg, just increase Hp. If it's that critical, put in a switch to bypass the alternator so you run at a total loss for a little while as you overtake - An extra 2-3Hp!!!!

Don't forget to switch back again though.
Edit - timmy201 may be right.