Having recently been on a White Water Safety and Rescue course, I then went to bed and forgot a good bit of the ropework (also the weather on the day of the course didn’t really help). Anyway I have been looking on the internet and nobody seems to have put together a guide for the 4:1 mechanical advantage system that you are taught so I have put one together. Click on a picture to enlarge it.
Diagram of the Setup
Pictures of the Setup
Once the karabiner on the end of the throwline meets the anchor point the throwbag can be pushed back towards the pinned boat and the line can be pulled again. This should be repeated until the boat is free.
This picture just shows the whole system, the throwline attached to the end of the boat would be the one that the open boater has pulled when they have swum with their boat.
If there was no line from the boat then the karabiner should be clipped into the boat. This is less ideal because it requires someone to wade into the river putting them at risk, however it will be the main way of attaching it to a pinned kayak.
A more efficient system is a pig rig, the system is essentially the same but the karabiners are replaced with pulleys (they can be with this if you have them), and prusiks are used to prevent the rope slipping backwards.