Can be done two ways. You could do it figuratively, meaning that you had gone well out of your way for someone or some cause as a way of showing your love or devotion. Metaphorically, it can be an implicit promise between two people, meaning one would do nearly anything at the whim of the other.

Or you could be a badass and do it literally. Here is how:

First you will need a nice healthy bonfire. The firepit should be at an even level with the ground around it and, ideally, newly created for the specific purpose of the current bonfire. The base of the fire should stretch at least three feet across between some two points on the fire's perimeter. Try and build the fire out of wood with no nails or other pieces of sharp metal embedded in it. If this is not possible, have a large magnet handy. Do not use any incendiary agents to start the fire that will leave a residue harmful to the skin.

Let the fire burn to its peak, which should be at least five feet in the air for a good half hour, if not higher and longer. Then let it die unaided, as throwing water on it or other such measures will decrease your net supply of hot coals.

Once there are no more flames, begin to rake the coals into a strip about a foot wide and three inches deep, and as long as you can make it. Again, if flames jump up as you're raking, stop and let the fire die down a bit more. If you had to use wood with nails or staples in it, run a large magnet down the length of your strip until you can bring it up with no more metal attached to it. The inside of an old speaker works well for this. Believe me, it is no fun having red-hot metal stabbed into your foot.

If you've done all this correctly, you should have a strip of coals that is ready to be walked across. As a final safety measure, check the color of the coals. They should be a little less than half black, but there should still be a lot of red and even some orange in them. If you pour a little water on the outside perimeter, it should sizzle and evaporate almost instantaneously. If a puddle forms, or your coals are mostly black, you have waited too long.

Now you can walk. At least your first time, it will behoove you to lightly coat the soles of your feet with mud. If no mud is readily available, you can make your own by adding water to dirt. Roll up your pants and walk around in it for a while. It helps if the mud is at both ends of the strip of coals, thus serving the dual purposes of coating and cooling. Do not use water - it will evaporate afer your first couple of steps.

Steady yourself and walk, do not run. Place your feet down as evenly as possible, as digging in with the ball or heel of your foot will push it down into the coals and may cause it to burn. Likewise, try not to curl your toes, as you may inadvertantly carry some of the coals with you, in a sensitive area. Keep your steps light and quick, but remember that running will cause you to step with more pressure. Your feet should feel warm and may tingle a little as though you were immersing them cold into hot water, but there should be no pain. If at any point in walking across you experience pain that is more than a tingle, immediately step off to the left or right of the strip and go place your feet in the mud. The idea is not to hurt yourself.

Note: It would be really, really smart to try this the first time under the supervision of someone who has done it before. Also, kids should not try this, as their little feet are softer and they tend to step more heavily.

That's it. Be safe, have fun.