19-back is a not uncommon cribbage variant, but is often unpopular. I've seen it in some editions of Hoyle's Card Games; I've had 3 editions, read a fourth, and 2 of them had it. Unfortunately, my copies are not available to check exact wordings at the moment.
If your hand scores no points in count phase, you lose 19 points. Note that this doesn't apply to the crib, only the hand, and only applies if you get to count.
Some people understand it to be:
If, during your turn, you score no points in play nor from hand during count, you lose 19 points. This applies at point of pegging the hand, before going to the crib. Again, if you don't get to count, it doesn't apply. Note that this means the dealer can't 19-back, as a dealer ALWAYS gets at least 1 point in play (either a 31, go, or last), and why this is not the usual version of 19-back.
The naming is in part because 19 is impossible to achieve in the scoring phase. It is not uncommon for a 0-point hand to be called a "19"... since experienced cribbage players know that 19 is impossible.
Having played it, it reduces risk taking in card selection, due to the need to ensure points in scoring. Note that it also tends to reduce crib scores a bit as well, as the dealer has to make suboptimal decisions in order to keep points in hand. Also, it penalizes (pretty heavily) a "playing only hand" that fails to take one to 121+.
A playing hand being a low-point hand with strong potential to get points from by-mees or expected runs and doubles, such as AA22 (goes for the Go points) or 55AA (for getting 15's) or AAAF (F=any face; goes for the goes and pairs at high count). A Playing Only hand has no points itself, but expects to get them in play, such as A2TK or A389.