Describe the knowledge at the level of function and history rather than formula, and support it with document indexes, redacted excerpts, training logs and declarations from supervisors who can be contacted. USCIS needs to understand what the knowledge is and why it matters, not to reproduce it.
Show the shape of the knowledge, not its contents
Prepare a short technical narrative written for a non-engineer: what the process achieves, when the company developed it, which products depend on it, and how long it takes to train someone to use it. Attach an index of the internal documents that exist, with dates and titles, and include redacted pages that show structure without exposing parameters. Add the employee's training records, project assignments and any internal certifications, together with a list of colleagues who hold the same knowledge, if any, which also helps explain why this person must be the one to go.
A declaration from the shop's owner or engineering lead explaining the employee's contribution carries weight when it is specific. Keep confidential material behind a confidentiality note and be prepared to provide more if asked, rather than withholding evidence the file depends on.