An impact wrench needs something solid to work against in order to generate the high torque for bolt loosening. It cannot generate this torque against something loose or springy (like your hand). The hammer will stay locked onto the anvil and the wrench simply rotates like a drill. Only when it encounters solid resistance (like a lug nut) will the hammer disengage from the anvil and start delivering multiple high speed rotational impacts against the anvil, exchanging its momentum for high torque at the drive end.
In my opinion, it sounds like a discharged battery "it's getting weaker and weaker and weaker".
Good Luck