why is the enhanced for loop more efficient than the normal for loop

Question

I read that the enhanced for loop is more efficient than the normal for loop here:

http://developer.android.com/guide/practices/performance.html#fo

Answer 1

From Effective Java :

The standard idiom for looping through an array doesn’t necessarily result in redundant checks. Modern JVM implementations optimize them away.

But Josh Bloch doesn't describe how JVM optimizes them.