Complex sentence: What Microsoft didn't realize is that the clock that counts down the time to the checker can be destroyed by un-linking it to the kernel checker, therefore the kernel checker will never receive the command to check, and leaving the kernel vulnerable to hacks and rootkits.
Split into simpler sentences: What Microsoft didn't realize is that the clock that counts down the time to the checker could be destroyed by un-linking it to the kernel checker. This would make it so the kernel checker would never receive the command to check, leaving the kernel vulnerable to hacks and rootkits.