over extended would be trying to be a human brigde and realising the gap you wish to cover is to large but you have no leverage to get anywhere but down because you are ....overextended.... the only box there ever is is the one you create for yourself
I'm assuming you mean the joints in the human body here... Overextending is pushing them past their normal range of motion. Leaving your "box" as it where can cause joint damage, and can hurt a fair bit too. Some MA schools encourage you to overextend the technique by fully locking out your arm/leg. This shouldn't be done, as it leads to long term damage.
Do you mean over extending on a strike? Like leaning forward to punch a target that is a little too far away because you misjudged the distance?
I think in general that you should keep a limb slightly bent at the joint as if it's straight you'll damage your muscles and possibly your joints.
