These are some helpful demos for the underlying tech behind Be Right Back. They're not perfect, but they're a good starting point for getting a feel. All of these will be referenced by the blog articles as well.
Type a message and watch it become the NSArchiver 'streamtyped' bytes Apple writes to disk, then watch the decoder pull the text back out.
Drag the window and stride sliders and watch overlapping chunks form over a real conversation, with live overlap and coverage stats.
Click terms to highlight matching documents. Type queries to see AND/OR postings-list intersection.
Drag tf, k1, b, and document length and watch the BM25 score - and its saturation and length-penalty curves - respond live.