The documentation for this part of YARV is pretty poor. I am not sure if this is because the only developer who really understands this does not speak English or does not intend to, because this part of YARV is still in a difficult development.
It’s best to use the source, ask questions on the ruby kernel mail list, or (for best results) learn Japanese and ask questions on the ruby-dev mailing list.
The fact is that before the bytecode verifier (à la JVM) works, the developers really do not want you to mess around with the bytecode too often because you manually created illegal or “funny” bytecode sequences (t .e. not created by the YARV compiler), you can collapse the virtual machine (or, even worse).