Special sections in Linux binaries

I am sorry to bother, I would like to know how the alternatives mechanism works. Do you have more detailed information? Or can you introduce it?
I question that the alternatives mechanism means when the old instruction is accessed, then the corresponding new instruction position is executed by the alternatives mechanism, or the old instructions has been covered by the new instructions? Thank you by the way.

（很抱歉打扰了，我想请问一下替代机制是怎么工作的？我很好奇，它是在执行到旧指令的时候再去跳转到新指令的位置处去执行呢，还是系统将新指令拷贝到了旧指令的位置处，覆盖掉了旧指令这样的？谢谢！）