If pistons are numbered (1) (2) (3) (4) (5) (6) in a straight 6...
pistons 1 and 6 go up at the same time, and 2 and 4 go up at the some time, etc. thus creating equalibrium, and balancing of the engine as a whole.
V6/8 tend to rock slightly, and to stop this, additional mechanisms are put in place.....