Background: The correct positioning of the mitotic spindle during the asymmetric division of the nematode C. elegans zygote relies on the combination of centering and cortical-pulling forces. These forces, revealed by centrosome anaphase oscillations, are regulated through the dynamics of force generators, related to mitosis progression. Recently, we have presented the control of oscillation onset by the posterior spindle pole position in related species C. briggsae, necessitating a re-evaluation of the role of astral microtubules dynamics. Results: After exhibiting such a positional switch in C. elegans, we mapped the microtubule ends at the cortex and observed a correlation between the proximity of the centrosomes and the density of microtubule contacts. To explore the functional consequences, we extended the "tug-of-war" model and successfully accounted for the positional switch. We predicted and experimentally validated that the control of oscillation onset was robust to changes in cell geometry or maximum number of attached force generators. We also predicted that the final position of the posterior centrosome and thus the spindle has a reduced dependence upon the force generator dynamics or number. Conclusion: The outburst of forces responsible of spindle anaphase oscillations and positioning is regulated by the spindle position through the spatial modulation of microtubule contacts at the cortex. This regulation superimposes that of force generator processivity putatively linked to the cell cycle. This novel control provides robustness to variations in zygote geometry or detailed properties of cortical force generators.