The plot so far
At the end of our first two tutorials, we have:
 

Figure 2.1. Locating the Slap stroke

Fault tolerance
Strictly speaking, fault tolerance means an ability to cope with error, but I would like to extend it to include “being able to continue functioning under sub-optimal conditions”. Timing your second step to coincide with the slap helps you to do just that.

• Let's take as an example, three possible classes of time interval between each step: one that is accurately one beat long, one that is faster (by 5%), and one that is slower (by 5%).
• Let's also take into account whether a partner is dancing with the first step aligned with beat one, or the second step aligned with the slap stroke.
• To keep things crude and simple, let's assume that the steps are of uniform duration within each class.

Figure 2.2. Fault Tolerance

Looking at the steps aligned with beat 1, we can see that the error accumulates such that, by the third step, the error in duration is doubled.

Looking at the steps aligned with beat 2, we can see that each error is pushed to its respective edge of the step cluster, preventing them from accumulating i.e. an error in the first step is located to the start of the first step; an error in the third step is located to the end of the third step. These errors are absorbed by the nul beat of the double open tones where they cause the least harm.

If you really must know…
We can also gauge the net difference in timing (%) between partners with dissimilar step durations:

• Both aligned to 1
  Acc. // Fast: 0, 5, 10
  Acc. // Slow: 0, 5, 10
  Fast // Slow: 0, 10, 20
   
• One partner aligned to 1, the other to 2
  Acc. // Fast: 5, 0, 5
  Acc. // Slow: 5, 0, 5
  Fast // Slow: 5, 5, 15
  Fast // Acc.: 0, 5, 10
  Slow // Acc.: 0, 5, 10
  Slow // Fast: 5, 5, 15
   
• Both aligned to 2
  Acc. // Fast: 5, 0, 5
  Acc. // Slow: 5, 0, 5
  Fast // Slow: 10, 0, 10

Therefore the maximum net difference where:

1. both partners are calibrated to beat 1 is 20%;
2. one partner is calibrated to beat 2 is 15%;
3. both partners are calibrated to beat 2 is 10%.

It is the maximum net difference that's crucial. Dancers tolerate a certain amount of variance between themselves and their partner. As a matter of fact it's often desirable to have some variance since it adds colour to a partnership - try dancing to a metronome and you'll understand. So long as the net difference remains below a threshold level, a couple can re-establish dance synchrony during the double open tones. However if the net difference exceeds this level, partnerships will fail to re-establish synchrony.

Having at least one partner calibrated to the slap stroke (i.e. you) reduces the maximum net difference, and therefore increases fault tolerance. Ideally your step duration would be accurate as well, and that is a topic we shall address in due course.