CHA CHA is one of the hardest dances to understand and dance to the music. The conflict seems to be the timing of the CHA CHA steps. Notice that I did not say CHA CHA CHA!!
I have never heard anyone dispute that the musical timing of CHA CHA is 1, 2, 3, 4, &. Three whole beats and two half beats equaling a total of four beats (figure 1c). There is, however, a conflict because some dancers and instructors say "CHA CHA CHA" and others say "CHA CHA".
Let's analyze a CHA CHA pattern with the timing of 1, 2, 3, 4, &. The music is written in 4/4 time (fig. 1a) and the speed of the music is 30 mpm (measures per minute). This would make each measure 2 seconds in length (fig.1b). The ONE, TWO, THREE are equivalent musically to three "¼ notes" and the FOUR, AND are equivalent musically to two "1/8 notes" (figures 1a and 1f). The "¼ notes" would then be one-half second in duration and the "1/8 notes" would be one-fourth second each (fig.1e). I will compare a drummer and a pianist playing two measures of music and a dancer moving to the same two measures.
The drummer would strike the drum at the start of each "¼ note" and at the start of each "1/8 note" (fig.1d). The duration of the drum strike is measured in milliseconds (duration depends on the type of drum, tone, etc.). In percussion there's no ability to sustain a sound, therefore, a drum note of a specific length actually means striking the drum at the beginning of the note and doing nothing for the rest of the notes' duration.
The pianist (fig.1f) "depresses" () a key to begin each of the three "¼ note" tones and "releases" () the key one-half second later to end each "¼ note" tone. The pianist would do the same for the two "1/8" notes except for depressing the key and holding it for just one-fourth second per "1/8" note tone.
The definition of a CHA CHA (Latin/Rhythm) "step" in ballroom dancing is: "A complete weight change from one foot to the next but the last foot remains in place". The weight change (step) is initiated at the beginning of the note (on the drum beat) and is not completed until the end of the note. Commencing with the left foot (fig. 1g), the ballroom dancer would start moving the left foot on the drum beat at the beginning of the "¼ note" (at the same time that the pianist "¼ note" tone starts). The step is complete when the weight is moved to the new foot (the last foot remaining in place) at the end of the "¼ note" when the tone ends one-half second later. The same would be true for the next two steps (¼ notes) the Right and the Left (fig. 1g). The dancer would start moving the right foot on the drum beat at the beginning of the "1/8 note" (at the same time that the pianist "1/8 note" tone starts). The step is complete when the weight is moved to the new foot (the last foot remaining in place) at the end of the "1/8 note" when the tone ends one-fourth second later. The same would be true for the next step (1/8 note) the Left (fig.1g). The above is true for the second measure of music except the dancer would start with the Right foot.
The ballroom dancer must dance the same way a piano player uses the keys to play notes. As the key is depressed the dancer starts to move. When the piano player releases the key, the dancers step (weight change) is complete. Using our example, let's compare the drummer to the dance movement. You can think of a drum strike as a starter pistol. The starter pistol starts the race just as the drum beat starts the dance step movement. The drum strike (fig.1d) at the start of the "¼ note" would start the dancers "¼ note" step and the drum strike at the beginning of the next note would complete the "¼ note" step and simultaneously start the next step. This would be repeated for each "¼ note" and each "1/8 note". Even though the drum strikes start different notes, they effectively start and stop each note and step because the drum strikes have very little duration.
There is a misconception between "musical beats" and "ballroom dance movement", especially when dancing CHA CHA. Dancers hear the "CHA CHA CHA" sound in the drum beat rhythm as indicated by the circled portion of fig.1d. The dancer can not complete a weight change as fast as a drum beat. This is why dance steps relate to the piano player (because there is duration of the tone that allows time for the step).
You can clearly see by the circled portion of fig.1c that each of the first 2 CHAs (½ beat each) have a shorter duration that the 3rd CHA (1 full beat). When dancers say "CHA CHA CHA", they tend to give all of the CHAs the same duration, which will cause them to dance a half beat too fast. It just makes more sense to dance CHA CHA ONE because it describes the actual rhythm and will cause the dancer to use the whole duration of the "¼ note".
The ballroom dancer has to develop the ability to understand how to listen to the music and move accordingly. Once you can achieve this, the CHA CHA will feel fast and sassy but still in control and with the music.