The beat works by displacing the opponent's blade. However, the contact with the opponent's blade actually achieves this in two ways: (1) by giving the blade a direction of movement and (2) by transferring energy to the previously relatively stationary blade. The unintended consequence is that the beat imparts movement, and, in doing so, overcomes the inertia of the blade sitting in a guard. This movement vector can either be arrested by counteraction (a beat back or stop thrust or cut) or be continued.
The opponent who chooses to continue in motion can do two things that bode ill for the beat-straight thrust or straight cut attack. First, the displacement is converted into a circular motion at the hand and wrist, and that circular motion into a change parry. Second, and simultaneously with the change parry, the opponent executes a measured step back to give the parry time for completion.
Note that I use the term change parry as opposed to a circular parry or counterparry. Over the last 100+ years the meanings of the terms circular parry and counterparry have changed a number of times back and forth. Some even incorrectly use the term counterparry to describe the second and subsequent parries against counterripostes. In reality there are two circular parrying movements which look exactly the same, but which achieve different tactical ends:
(1) a circular movement which picks up a blade that has disengaged from one line into another and returns it to the original line. I term this a circular parry. For example, a point disengaging from 6 (or 3 in sabre) into 4 is met by a circular 6 parry and returned to 6(3).
(2) a circular movement which picks up a blade attacking in one line and moves it to another. I term this a change parry (much as a change of engagement picks up a blade in one line and returns it to another, or a change beat changes from one line to beat an opponent's blade from another.
So the change parry executed in response to the beat is a circular motion to take the blade. And how do we otherwise defeat a circular attempt to take the blade? The time honored answer is a counterdisengage, a simple attack by circular movement in the same direction as the taking or parrying movement. This is also the basis of the feint straight thrust-counterdisengage and the double attacks in foil and epee.
One answer in all three weapons to the change parry response to the beat is the counterdisengage to defeat the parry. In foil and epee this is effectively a compound attack. In sabre it is also a compound action, but it comes in two versions, as a point action off a beat intended to set up a point thrust (a somewhat unusual action), or as a conversion of the beat-cut from a cut into a counterdisengage point thrust. This is not a common sabre action today, but it is in the traditional sabre skill set and deserves practice for this specific tactical application.