Ok, hopefully this explains it enough, the construction key will have all but 1 or 2 cuts the same as the owners key, in the cuts that are different they will be cut slightly deeper, I measured the diameter of the construction ball bearings I have and they are .44 of an inch which is the same as a #3 master pin.
For this I am going by Lockwood depths, so say the owners key cuts are 36513 and the construction key is 34313, the ball bearings would be in the second and third chamber, when the construction key is used the ball bearings stay below the shear line, once the owner uses their key the ball bearings are lifted above the shear line and when the lock is turned they fall into the small hole so if someone tries to use the construction key the ball bearings would no longer be in the pin stack to make up the #3 depth for it to work.
All the ones I have found have 3 bearings in them, in my picture there are only 2 in the first chamber, I must of droped the third one somewhere, and just below the pins you can see the small holes for the bearings to drop into.
Hopefully this helps you if not I should be able to help you a bit more.
