The drill collar moment theory states that when a bit drills from a soft formation to an inclined hard formation, it will support a great portion of bit weight.
This causes a moment at the bit that tends to buckle the collar and deflect them from centre line of the hole. It may also shorten the active assembly length and can reduce any existing pendulum effect. The resultant tendency is up dip.
Now, it can be argued that when drilling from harder to softer formation, forces are reversed and constitute a down dip deflection.
Buckling and corresponding deviation forces increase with smaller size collars. greater annular clearance and high weight on bit. As more bit weight is applied, buckling increases.
The length of assembly between bit and first point of drill collar to wall contact, tends to shorten. This section is called active drill string length. Usually not more than 150 ft (45m) of assembly affects it's deviation characteristics and with build up assemblies using high bit weight probably less than 25 ft (7.5 m) is involved.
The active drill string length is usually determined by the position of the first full gauge item in assembly above the bit (reamer or stabilizer). When the distance of bit to first stabilizer or reamer is short (5 to 12 ft), buckling will be induced in next drill collar above. The full gauge tool becomes a fulcrum or pivot point. The shortened effective length increases the angular difference between axis of hole and axis of assembly, and promotes hole deviation.