The context for this play is that your opponent is just out of reach. Depending on the translation, either you literally can’t reach him or it is merely hard to do so. For our purposes, lets say you can barely touch your opponent with a passing step.
First Tempo: A Beat from Fourth
Assuming you are in a wide Quarta (fourth), you begin by disengaging under his sword in order to beat it to your right. The disengage and beat must be done as a single action to avoid giving away your intention.
Second Tempo: Passing into Second
As the beat is concluded, you immediately pass to the opponent’s right side with you left foot. (Agrippa is sidesword, not rapier, so the passing step is still a commonly used technique.)
As you make this passing step, raise your hand into Seconda (second). This isn’t necessarily meant to be an attack, though it may very well land the important thing is to be in the correct counter-posture for what happens next.
If he hesitates…
If the blow doesn’t quite reach, continue your movement with a second and third passing step. Since you don’t control his blade, continue on past him so that he must turn to present a threat.
If he parries…
If your opponent parries by pressing his sword into yours (forza, which is similar to a bind), stay in second and be quick to grapple. I prefer to reach under the swords (Agrippa’s second is shoulder-high) and grab his hilt.
If he jumps back…
If your opponent jumps back to break measure, his sword is going to be either to your inside or your outside.
If on the outside, stay in second, pass with the right foot towards your right and thrust. If on the inside, lower the hand into fourth, pass with the right foot towards your left and thrust. Stepping into his blade will help to push his off-line.
What if the beat missed?
The astute opponent may counter-disengage to avoid the beat. This is acceptable for two reasons.
First, the goal of the beat was to distract the opponent. You don’t really care whether or not he retains the original center line, for your passing step moves the line. In fact, the harder he tries to keep his sword in the original position the less time he’ll have to react to the second tempo
Second, he is too far away to hit you. Assuming that you are of equal stature, you wouldn’t try this technique if he could easily offend you during your disengage and beat.
What if I am slightly out of reach for the beat?
The text seems to say that you can perform the first passing step and the beat as a single action. I think whether you do this as one tempo or two depends on the context and isn’t crucial to the overall theory behind the play.