[octachron]

Ok, now I understand: this part is a description of how optimizing C bindings using the OCaml FFI often requires to play around the GC:

With OCaml uniform representation of data, the GC will follow all data that looks like a pointer in its reachable set of values. Which is the right thing to do for valid OCaml values, for which anything that looks like a pointer (outside of strings or numerical arrays) is a pointer to a valid OCaml value.

However if you are building OCaml values in the C FFI, and that this in-construction value is somehow reachable by the GC, you have to make sure that there is no pointer in the uninitialized part of the data. Or alternatively you need to make sure that the runtime will not enter a GC phase while you are building those values.