I'll add a little to Mark's excellent answer. Most Packards have a pin that selects for instant and bulb. In bulb mode it is very responsive to negative air pressure. So you squeeze to open and release the bulb to "suck" it closed again. It's possible to get about 1/8 second doing this. Slower is simply holding the air pressure a moment longer before the bulb sucks it closed. With practice you can listen to a Nikon FM's 1/4 second and get a perfect 1/4 with the packard. Same for 1/2, 1, etc. In instant, a hard squeeze will give a very repeatable 1/25th 1/30th. An easier squeeze will give a longer exposure. I've grown so comfy with them that it's rare indeed for me to be spoiled these days with a modern shutter.