XORification is the technique to make a Boolean function or formula harder by replacing every variable $x$ by the XOR of $k\geq 2$ distinct variables $x_1 \oplus \ldots \oplus x_k$.
I am aware of uses of this technique in proof complexity, mainly to obtain space lower bounds for resolution-based proof systems, e.g., in the papers:
- Eli Ben-Sasson. Size space tradeoffs for resolution. STOC 2002, 457-464.
- Eli Ben-Sasson and Jakob Nordström. Understanding Space in Proof Complexity: Separations and Trade-offs via Substitutions. ICS 2011, 401-416.
Are there other uses of this technique in other areas?