The mechanisms of collision induced fragmentation of small Ar-n(+) (n=2-9) clusters are investigated in the 100 eV center-of-mass energy range. The velocity vectors of the fragments are measured in a multicoincidence experiment for two- and three-body fragmentation. The relative role of the two basic dynamics, electronic transitions, and momentum transfer in binary collisions is evaluated. The structure of the clusters deeply influences the type of mechanism. This is clearly the case of Ar-3(+) for which a specific impulsive process called "diatom" mechanism plays an important part in the fragmentation of one isomer.