If you're thinking about the Watson/Computrol type loader, the film comes from the supply drum through a light tight gate and in to the cassette loading chamber. In daylight, you open open the door to the cassette chamber, and there will be an inch or two of film leader there. Open your cassette and tack the inch or two of film coming out, on to the cassette spindle, close the cassette, and put the cassette back in the cassette chamber so that the crank engages where it should. Then replace the light-tight door to the cassette chamber, and crank until you've turned in the number of frames you want, plus a few extra to allow for the leader you're going to trim in the usual way. Open the cassettre chamber, cut the film, remove the cassette, and trim the leader.
Originally Posted by fotch
There's an interlock with the cassette chamber door which means you can't open it until you've turned the cover on the supply drum in such a way that the gate inside becomes light-tight, then you can open the door to the cassette.
Because there's always an inch or two of film poking out from the supply drum to the cassette chamber, that short length will be wasted, and will form the last couple of frames when the cassette is being used in the camera. Normally, you'd add an extra couple of frames to your count, say, 38 instead of 36 (plus film leader on top of that) to allow for it, and just make sure never to go past the '36' on your camera's film counter. In my early days, I made just that mistake. I got into the habit of putting the loader into a changing back whenever I had to open the cassette chamber door, to avoid having exposed film at the end of the strip. I got used to doing that, though I can understand some wouldn't want to be bothered with that inconvenience.