gc vs. no gc?

in the original paper it mentions being able to write (pre?)Scheme
code with a GC and then decide what parts to then take over for manual
memory management to get better performance in key places. has anybody
done this? does it mean starting with Scheme48 and then going back to
PreScheme or something?

i didn't see or missed the part where it explained how that would
really work since it say there is no gc at all in PreScheme.

thanks! (pre scheme is very exciting to a nerd like me.)