From transcript of a space mission:

[Cernan - "At ignition, the tanks were about three quarters full of fuel and one quarter pressurized helium. The trouble was that, in zero gravity, you didn't know where the fuel was in the tank. Therefore, you had to apply a little acceleration and drive the heavy (that is, denser) fuel downward towards the outlet, towards the engine, so that, when you started the main engine you don't get a fistful of helium. Because we were flying with the landing gear and the main engine forward, to get proper ullage we had to fire the RCS jets briefly against that forward motion. We had the computer programmed to fire the thrusters for a few seconds prior to ignition; but, if Jack hadn't confirmed ullage, I would have used the handcontroller to give us manual ullage."