Okay, here's how it works: The background behind the players name and class is now a bar representing how many hit points you have left. The colours are as follows:
| Hit Points Remaining | Bar Colour |
|---|---|
| 100% | Green |
| 50%-99% | Yellow |
| 25%-49% | Purple |
| 24% or less | Red |
The blue portion of the bar shows how much damage you took in the last hit - for example in the screenshot above I took quite a bit.
The bar can be turned on or off in the options menu, and controlled by "hitpointbar" in your NETHACKOPTIONS.
With any luck it should not cause your game to crash or anything... I've ascended twice while using this patch without any problems, but you never know.
Download the hitpointbar diff for 3.3.1 sources here.