These are used for sea kayaks - I've never known such a thing on a whitewater kayak.
Many paddlers use PFD towing belts for towing other paddlers. Others still use webbing or rope loops around themselves or the coaming. Both those techniques have advantages and disadvantages. Another approach is to use a deck-mounted towing attachment.
If you wish to tow someone with a deck-mounted rig, it is important to ensure that the attachment point is as close to the centerline of the kayak as possible. As well, it should be as close to the cockpit as possible. Otherwise, the towing forces will result in tracking problems for the kayak.
In order to make it easy to position the towing line and provide a quick release mechanism, two components are used to make the towing rig work. On the centerline, as close to the rear of the cockpit as feasible, mount a fairlead. On one side of the kayak, just behind the cockpit, mount a cleat to hold the line. The towing line can be run through the fairlead and then to the cleat. This keeps the tow line centered while providing easier access to the cleat to release the line.
Several types of cleats can be used, however, cam cleats are generally preferred. You can get a cam cleat with a fairlead on the cleat to help prevent accidental release if the towline is pulled up.
Some paddlers, especially whitewater paddlers, prefer to use a pigtail style contact tow system. However, in sea kayaks, deck-mounted contact tows are often preferred. Due to the potential for longer distance towing in sea kayaks, a deck mounted system allows the paddler to observe the towed person if arranged appropriately.
For a contact tow, simply mount a low profile cleat to the foredeck in front of the cockpit. Then use a short (approx. 1.5 meter) length of line with a carabiner or other closed hook on one end. Run the line under your decklines and through the cleat. The carabiner can be used to hook onto the other paddler.