Booting kernel from fastboot

We all have heard of fastboot boot xxxx command. How does it work when you use it for the kernel (fastboot boot boot.img)? 1. Does it boot Android with that kernel if possible, or does it make that kernel permanent? If not, how does it work? 2. Does it just put the kernel you specified in RAM instead of one in internal memory? 3. Also, what happens if the kernel can't boot for some reason? Does it soft-brick the device or just boot the existing kernel? 4. And if the kernel is ok and it boots, how to get back to using the kernel from internal memory?