[plt-scheme] getting better speed with JIT

Will Clinger recently posted in comp.lang.scheme on the speed of various 
Scheme implementations, and mentioned in passing that one has to 
(require mzscheme) to get the JIT to work. I was not aware of this. I 
cannot find this incantation anywhere in Help Desk, except in the mzc 
manual. Should I have known this, and if so, what did I miss? It sped up 
one of my benchmarks, a double loop looking for perfect numbers, by a 
factor of 2.5. Thanks. --PR