They caught this one during the day.
If you look closely, you can see he is "sleeping."
I myself have never actually had the nerve to catch these wonderful sea creatures. I am much more happy just to be the camera clicker. They, though, love it. I can't give you specific techniques. I can just tell you what I see. Catching a crab can take some time as they can be very quick. Usually one person keeps the flashlight on the crab while the other "chases" it. My daughter usually covers the crab with sand. Then goes to the crab from behind it, picks it up from the back placing her fingers somehow to get the crab to "sleep." The first crab of the season usually takes the longest to catch, but once the kids are back in the groove "catching" gets quicker.
I know this might not be an activity for everyone, but if you've never gotten to the beach for an evening walk with your children, this is something you might want to give a try. It has become a tradition for us--as crazy as that sounds. It is a part of our vacation that our children can not live without.