When replenishing a ship's supplies at sea, a method is required to transfer items or personnel from one ship to the other. This method is the jackstay.
Basically, a jackstay is a rope spanning the space between two ships. Attached to this rope by a block is a traveller, which is a hook or platform to carry supplies. Also attached to the traveller (this time by knots) are two lines that are used to pull it across.
The height of the jackstay above the water is determined by the sailors that pull on the upper rope. Sometimes, usually when a sailor first goes over the jack, but also on birthdays, a sailor will be dipped. In this case, the sailor thinks that he's going to the other vessel, and so willingly gets onto the jackstay. Once he is between the vessels, he will be lowered into the water. Of course, since this is a hazing ritual, it is officially banned. However...