On a conventional aircraft, the CG is normally in front of the center of lift. With this force arrangement the tail is required to create negitive lift in order to balance out the other two forces. With the CG and center of lift close together, it requires a very small force at the tail to offset the imbalance, while moving the CG forward requires much more. As these two main forces are moved further apart, the model has a greater static stability. The down side is that as the airplane slows down for landing, the tail may not be able to created enough down force to bring the wing to a stall (which gives the lowest landing speeds).

On the other hand, as the two main forces are moved closer together, the static stability is reduce, and at some point the control become divergent. In this case, the model is difficult to fly smoothly, and you find it to be very pitch sensitive. When very tail heavy, the model becomes impossible to fly.

On a trainer, a very nose heavy airplane will self recover in pitch when trimmed out. This is because when the airplane is gaining speed in a dive, the extra force on the tail pulling down will cause the airplane to return to a climb. If climbing it slows down which reduces the force and lets the climb go back towards level. A well designed and balanced airplane will go back and forth decreasing amounts until it returns to trimmed flight.

As the CG is moved back, these forces are reduced to the point where the airplane will no longer self recover.