I guess you're wondering just what that might be and I would also guess few ever do this, so what is it? Most skimmers are fitted with an air inlet pipe and or silencer and as you will know air is sucked into the skimmer via one or the other. Now along with air, the skimmer will also suck in dust and anything else in the air. Over time this can build up in the venturi and reduce the efficiency of your skimmer as it can build up inside various parts of the venruri. The answer to this is to attach a short length of pipe to the silencer, get some very hot water in a cup r mug and put the open end into it. The skimmer will suck the hot water in and through the venturi and cleaning it of crud, it may also help clean the impellor at the same time. If your skimmer simply has an open ended pipe to the venturi put the end of this into the hot water, if it has an air filter remove this and replace on completion. This is worth doing every couple of months or so and will help maintain your skimmer at optimum efficiency.