Tumbao Moderno
The word tumbao is derived from the Spanish verb tumbar meaning: “to knock out” (see http://www.diccionarios.com/), hence a conguero or conguera “knocks out” a rhythm on the drum. The tumbao moderno is commonly found in salsa, and can be loosely translated as the “modern rhythm”.

Originally Yoruban (from modern day Nigeria), the tumbao moderno is played on tall hand drums called congas or tumbadoras. The drums themselves hail from the Bantu people of modern day Congo. The modern rhythm is played by a single drummer, and was formed by the amalgamation of two older rhythms.
 

Role of the open tones

Here's a familiar scenario:
Imagine a class full of salsa dancers, learning to dance to time. Music is playing and the instructor is vocalising a count over it, indicating the dance cycle:

“One — Two — Three — Four —, One — Two —…”

Every time the count swings back to "one", the more experienced dancers cope with the beginning of the cycle, whilst the newer students struggle to hit the beat.

Why does this happen?
The new dancers are reacting to the count of “One”. They wait to hear it, process it, and then issue their bodies with the command to move. By the time movement is effected, it's late.

The experienced dancers have learned to anticipate the count of “One”. They compensate for the time lag by issuing instructions to move before the count of one.

So how does this relate to the tumbao moderno?
The start of the dance cycle occurs just after the open tones. To put it another way, the open tones of the tumbao moderno anticipate the start of the dance cycle. By listening for them and preparing to move after they sound, you are letting the open tones cue your movement.

Figure 1.1. Open tones of the tumbao moderno

The period of step inactivity in salsa is not only important for calibrating your timing, it also constitutes a crucial part of the “dancer-as-percussionist”—rhythm dynamic. Think of it as a “call-and-response” pattern: for every call of the conga in the form of two open tones, you respond with three steps.