Apple loves to sell power supplies. Accordingly, the magsafe plug is designed to fail (short-circuit) with a half twist of the "cable to L plug" joint.
In order to invalidate any effort at self repair, there is a small controler in the magsafe plug that communicates with the "recharge circuit" inside the computer, and it is bloodily hard, if not impossible, to repair it without breaking some of the fragile circuits inside (notably the unprotected leds).
Now, if the computer detects that you have fiddled with anything, it will take the voltage to power the computer, but will refuse to charge the battery, effectively leaving you with no option but to plunk 79$ on the counter for another soon-to-die apple power adapter.
How grand is that...