Games that are protected by DMA "cycle" through different addresses, to keep from one "hack", or "code" to be able to effect that action permanently.

Basically, everytime you die, reboot PSP, or finish a match, the address for a code changes to something completely different...thus making your code...useless.

Not all addresses are DMA'd, but common one's that developers think will be targets to hacks - usually are.

There are ways to bypass DMA, but it's not very easy for beginners.