Role of Calcium and Magnesium in capacitation/acrosome reaction
The process is not clearly understood. Calcium helps in capacitation: During capacitation calcium enters the spermatozoa through ion channels; blocking these channels inhibits the acrosome reaction but what calcium does inside the spermatozoa is not very clear. This paper says that calcium activates a certain tyrosine kinase in the spermatozoa which in turn activates a protein called p32. This they say occurs "concomitantly with capacitation". Other papers cited by this article talk about involvement of calcium dependent adenylate cyclase. 

This article also says that protein tyrosine phosphorylations are important for capacitation but doesn't describe how.

I could not find a more precise information on this topic. I shall update the answer if I find one.