The cartridge has a center section that is sealed at the mouth end. That's where the filler and juice goes. On two sides, there are little slits - this is where the vapor comes through and out the mouth piece.
If you overfill the cart, little drops of juice can get into the slits while you are using it. Then the juice gets into your mouth and burns your tongue. The burning sensation is usually from the nicotine.
So blowing into the mouth piece forces the juice back out those slits. I do it with the battery disconnected because juice can build up around the battery connector and make a mess (or on unsealed batteries, kill the battery, but I believe yours is sealed).