Essential tension basically means there isn’t a single absolute solution to a problem. Instead there is a dynamical balance that needs to be maintained. A shift to one side of a spectrum needs to be counterbalanced by increased focus on the spandreled roots of what was removed. One of the ideas of essential tension is that freedom lies in the balancing of forces rather than the removal of problems.
Here is a small selection of references I think touch on this idea:
From a religious perspective
-J Bonner Ritchie on essential tensions between individuals and organizations
From the perspective of organizational evolution
-Willis, R. (2004). A complexity and Darwinian approach to management with failure avoidance as the key tool. In Complexity theory and the management of networks: Proceedings of the workshop on organizational networks as distributed systems of knowledge (P. Andriani & G. Passiante Eds.). Imperial College Press: London. pp. 74-88
From the perspective of knowledge construction
-Impossible knowledge - Haig-Brown, C. (2003). Creating spaces: testimonio, impossible knowledge, and academe. Qualitative Studies in Education, 16(3), 415-433.
-Difficult knowledge - Pitt, A. & , Britzman, D. (2003). Speculations on qualities of difficult knowledge in teaching and learning: an experiment in psychoanalytic research. Qualitative Studies in Education, 16(6), 755-776.