Nope, I'm a pretty inexperienced kayaker. I'd love to know what wash
diving
is! I'm one of these people who is annoyingly curious about everything!

The best way to see what happens is to watch someone in a short boat paddle
as hard as they can past you. If their boat is trimmed to keep the nose out
of the water, the stern dips. However what you watch is the series of waves
created by the boat. The obvious one is the bow wave. But behind the boat,
just rear of the stern is the transverse or stern wave,. As the boat speeds
up this wave becomes a "hole" behind the boat. The stern of the boat as it
dips, drags into the hole and gets held back. Hence every hull shape has a
maximum speed that cannot be exceeded no matter how hard you paddle.

You can get close to the boat in front and sit on the bow wave and surf
along, easing the load on your paddling. However, I find it difficult to
maintain position for any great distance. Alternately you come up behind the
boat in front an surf in his stern wave. This is easier, but you tend to end
up running into the back of the boat in front.

It is very similar in theory to riding a bike in another rider's slipstream.
I find it can give some releif from paddling, and can help slower boats keep
pace with faster boats - for a time.

At least that's my understanding.

Ewan Scott