You asked how to use LF (large-format) lenses on a Hassy. Is this what you actually mean? Or do you simply mean leaf-shutter lenses? If it's the former, you can't use large-format lenses on a DSLR like a Hassy. There's no adapter. Even if there were, you wouldn't be able to focus anything wider than an 80mm; those lenses need to be closer to the sensor/film plane than the Hassy body allows. And there's no way to slide them closer to / farther from the sensor for focusing. Even if you had them in helical mounts, as I mentioned, you couldn't get the wider ones close enough to the sensor. There's also no connection between the body and the lens, so when you triggered body's shutter, the lens wouldn't fire.
If somehow you were able to mount a large-format lens on a Hassy, I think you'd compose with the mirror down and the aperture wide open. Then you'd stop down the aperture, lock up the mirror and trigger the lens's shutter to shoot (with the lens connected to the back via PC sync, as on a technical camera).
Now, if you simply mean using leaf-shutter lenses on a Hassy, like the HC lenses, it's just like any DSLR. You fire the shutter from the camera, the mirror pops up, the rear gate on the body opens, the lens stops down and the shutter activates. It's pretty instantaneous, like any 35mm DSLR. There's of course a delay in nanoseconds between the moment of pressing the shutter and the image being captured, due to all those in-between steps the camera does, but it's nothing.
Hope that helps.