Here's how I understood it:

1) The data goes through many network hops from the client to reach the S3 server

2) Each server in the middle has to scan data twice without HTTP trailer

3) The servers receive the data and compute checksum

4) And re-compute the checksum before sending it to the next server in the path

5) HTTP trailer attaches checksum to the chunks and avoids it

Probably I misunderstood it.

So someone else could explain it better.