This technique is such a life saver. Have you ever come across a recipe that asks you to pound out a chicken breast or another cut of meat? It is an important step and is done to either help tenderize a piece of meat or done to even out pieces that might be thicker at one end, which would be the case for chicken breasts. Pounding with a mallet takes whatever piece of meat and sort of levels it throughout.
Pounding itself is not hard to do but it's one task that sends you to the shower immediately afterwards, because it is a bit messy and unsanitary. As you pound, little bits of meat go flying in all directions and occassionally you will feel little bits hit your face and body. Yikes! A lot of cooks will cover the meat with plastic film and then pound but I have found that plastic film is not heavy enough to withstand the pounding and it gets torn to shreds which can sometimes get worked into the meat.
So what works really well are heavy duty ziploc bags. Just slip the piece of meat that you are wanting to pound inside the bag and and you benefit in three ways; 1) You keep all the juice and bits that have a tendency to scatter, inside the bag 2) the heavy plastic can take quite a beating and will not tear during the pounding and 3) if the bag is of a high enough quality you can throw all your pieces of meat inside and then cover them with a marinade and use the bag as the marinading container. It's awesome!