Same swap the articles get — every consecutive pair of words trades its leading consonant sound. "Ham sandwich" becomes "Sam handwich." Type below, no article required.
Every /wiki/<title> page pulls the real lead section (and often the first heading) straight from Wikipedia's public API, then runs it through the same word-pair swap as the toy above, server-side. The spoonerism site did this first for a fixed word bank; Pikiwedia points the same algorithm at the whole encyclopedia.