The Bowline on a Bight provides a large pair of loops at the end of a rope. Tied in the end of a spinnaker halyard - provided the halyard is long enough - it can be used to recover a man overboard.
If the casualty is conscious he slips a leg into each loop and hangs on tightly to to the halyard whilst a crewmember hoists him back aboard. Males may well complain bitterly about the location of the loops and need to be reminded that it's better than drowning.
An unconscious person will have one loop slipped under his arms and his legs inserted into the other before being hoisted aboard.
A quick and easy knot to tie, the Bowline on a Bight is also easy to untie even after it's been put under heavy load.
Form a long bight.
Maintaining the long bight, form a loop in it as shown here.
Now pass the working end of the bight through the loop from back to front.
Now open up the section of the bight that has been passed through the loop.
Pass the opened bight over the rest of the knot until it finishes up encircling both standing ends.
Finally pull it tied. You've just made a Bowline-on-a-Bight!