Potential energy is energy that is built up and saved. When you are moving and letting out that energy, that is kinetic energy. You would be using the least kinetic energy when you aren't moving, which is at the top of the hill because you move faster the more you go down.
Science is based on theories and tests, if someone has a theory they will test and if it is correct yippie! and if it isn't they will redo the test over until they get it correct or unless it is unsolvable.