According to Rumelt (1979), "Rivalrous environments reward the exploitation of asymmetry and the exploitation of asymmetry, in turn, leads to the formation of niches and specialization as a means of adapting to niches. ...

The assumption of rivalry, therefore, allows the use of a powerful rejection rule in evaluating strategy: a strategy that does not either create or exploit an asymmetry constituting an advantage must be rejected (p. 203)."

Asymmetry refers to an "imbalance of resources between competing or adversarial sides in an economic or political contest, where the 'weaker' side takes unexpected advantage of nimbleness, agility, and leverage of new technology or globalization. The term can refer to military struggles (counter-terrorism), or to business (individual entrepreneurs competing with bureaucratic, regulated companies.)"

from Rumelt, R. P., "Evaluation of Strategy: Theory and Models", in Schendel, D. E. and C. W. Hofer, Strategic Management: A New View of Business Policy and Planning, Boston: Little, Brown and Company, 1979, pp. 196-212.
http://www.doaskdotell.com/refer/vocab.htm