This patchbay is full normal, therefore when you plug a jack at the front in A or D, the original signal at the back of the patch (between B and C) is broken and the connection becomes between A and B and between D and C.
By simply welding a sigle cable, you can make it half-normal.
This means that when plugged as insert for a console for instance (the send of the console insert in B and the return in C), you may tap the signal (acting as a Y cable) in A without breaking the flow. Or you may use it as a real insert by taping the send in A and returning it in D.
Picture here : http://1mage.fr/images/monarchhalfnorm.jpg
Note that this modification does not prevent to flip the card and use it as parallel patch.