Its all to do with "clave". if dancing it QQ "side" the rhythm falls in line with that ( the 2nd bar on "2" is ON clave,, the 6th beat ).
When mambo first was introduced, it was taught breaking on "4", which is the beat preceeding the conga slap on " and", which is a clave beat.. but.. the change to the backward position of breaking on " 2 " falls more in line with the actual clave , hence the reason for the side action which was translated (QQ ) into the back break of Mambo.
Its all to do with the " call and response " from Cuban Rumba ( folk style ) .
One also has to be careful because the the clave may change from 2/3 to 3/2 without notice .
The 2 most dominant Clave rhythms in use are the Guaguanco in 2/3
and Montuno a rev Clave in 3/2.
most salsa is written in the Guaguanco style and reversals to Montuno in 3/2 is a more "folksie " side of latin rhythms.
In addition there is Guaracha in 3/2 which is the faster form of Danzon/ Rumba .
All of these come under the " Son " banner . ( Guajira is the Cha cha form )..
hope I didnt confuse you !!
PS.. this may be of interest.. The 2 chain schools have a different approach to Rumba Amer. style F/A commence side QQ.. A/M Fwd S.
The national D.D. for F/A was John Monte ( I worked in NYC with him ) and his brother was Hugo Montenegro, a world class musician.. that may have some bearing on their interpretation of Rumba.