In Mouravieff, fusion is the process of forming a 'real' or 'permanent I' out of the multiple little I's which generally constitute man's personality.
The process is explained with the analogy of a vase filled with iron filings. In the default situation, each filing points in its own direction. Shocks may cause displacement and rotation of the filings, causing heat. This heat is elsewhere referred to as the struggle between yes and no. Shocks and internal struggle may in time heat the filings to be hot enough so that they melt into a single block of iron.