Once you’ve taken the Light Wound (-1D), your skill is (temporarily) B4, so you hand over 1 Helping Dice.
Similarly, if you you are helping an Ob 5 test, and your skill is currently B4 (due to wound penalties), you log a Challenging test.
It can be pretty hard to find those Challenging tests when your skill gets high, so getting your character (punched in the face a couple of times / drunk / wearing a helmet backwards / etc.) beforehand is an excellent way to get such tests!
Have a look at p488 (Wounded Dice), where it describes penalty dice being subtracted.
Also on p488 (Zero Skills), you’ll see that if a skill hits zero, you cannot test it / help with it / FoRK it.
It’s all a handy incentive to go adventuring whilst injured! Sure, you’re sporting that Midi (-2D) wound, but just think of those challenging stat tests you’ll be able to get!
Although, do please note p489 (Zero Stats = Incapacitated). A wounded character is that much closer to getting knocked out, as the moment one of your stats hits zero, you’re down.