I use this technique when the olives are too small to be worth trying to slit or split them. Or I use it when I have olives that are halfway gone from being green to starting to turn purple.
The beauty of this simple technique is that there is no preparing the olives and no need to soak for days on end. The drawback of this fill and forget method is that it takes a long time for the curing process to do its magic as there are no obvious outlets for the bitterness of the olive to leach out into the brine, as would be the case if you slit or split the olives beforehand.
You may need to wait upwards of a year for your olives to be cured to an edible state. Patience is definitely needed and most people think they have done something wrong when they try their olives and they're still bitter. The only thing they've done wrong is not wait long enough!
GREEN OLIVES IN A SIMPLE BRINE SOLUTION
Other issues you might come across is black/grey/green mould on the top of the solution when you open the jars. This is telling you that the preserving didn't work and PLEASE discard these soft and mushy olives immediately.
You may also find that the jars, once opened for the first time, start to fizz. Don't panic! This is a by-product of the fermentation process. Have a look at this to see what it looks like.