and here's a picture that shows it.
For this process you can do one of two things: one is to bring the whole blade up to heat and quench it to harden, then use a torch or other heat source to heat it from the back so that when the edge section is at the desired hardness the back will be a bit softer; or heat the blade so that only the section you wish to harden is at full heat and then quench it, like this:
The effect is very visible here, with a fairly sharp demarcation between the fully hardened edge section and the softer back. I do one other thing to add to the effect, which is to hold the blade into the quenching oil almost horizontally so only the lower half is in the oil. You do move it up and down just a touch until it's lost most heat, then go ahead and lower it all the way down*.
After tempering this gives an edge that's hard enough to hold an edge well, with a back that's more of a soft/springy hardness; makes a blade that's very tough and flexible.