I would suspect anyone over the age of five is familiar with a diving board. A springy plank meant to propel the swimmer high into the air for a splash landing in the pool. Looking at a diving board, it's a simple and ancient design, essentially a lever and fulcrum.
A lever is arguably one of the oldest tools known to humankind. By setting a sturdy tree limb atop a rock, placing one end of the branch under the object to be moved and then applying force to the opposite end of the limb, the force exerted can be multiplied. In this way, large, heavy objects can be moved.
Revisiting the diving board, the forces applied at the opposite end of the board are then huge. A 200 pound man bouncing on the board would be creating several times that force on the opposite end of the board past the fulcrum. The very same multiplication of force can be found in structures. A great example is a deck.
Many decks are built using posts and beam to support the outer part of the structure. Typically the deck is cantilevered over the beam.
Can you see a similarity to the lever and fulcrum and the diving board?
With decks and any structure that is cantilevered, there is a major structural strength parameter that has to be considered;
The distance the structure can be built past the beam. The wood joists are only so strong, so depending on the species, grade and dimensions of the board, a specific amount of overhang is allowed.
In the picture above, the joists are 2" x 8" milled of southern yellow pine, the common species used in pressure treated wood found in Connecticut. Span charts for this species state that a 2' 8" cantilever is the maximum allowable distance. Most deck building specialists and internet sites do not recommend exceeding two feet no matter the size and species of wood. The main reason for this advice is the amount of stress that can potentially be exerted on the opposite end of the board at the house connection. Remember the deck is a lever.
While the cantilever is the short end of the lever, the force at the house connection is redirected as weight is applied past the beam onto the overhang. Instead of a downward force from good old gravity, the stress at the ledger board is altered becoming upward. As such this magnified up and down action can weaken the connection, especially when, as is so often the case, the fasteners are inadequate.
Decks most often fail at the ledger board.
Inspecting a newly built deck recently with an already horribly constructed stairway, the diving board like distance of the cantilever grabbed my attention. It was immediately quite apparent the length was well in excess of any framing and deck building parameters.
Look at the decking boards atop the joist. There are seven board plus from the where the beam is placed. The width of these boards is 5-1/2". That would equate to about 40". That's over three feet! Further look at the beam end. It does not support the outer joists. Instead the DIYer used his nail gun to pop about 10 nails into the post that supports the beam. The opposite end was "supported" exactly the same.
More nails does not make the connection stronger, it weakens the wood.
Unlike the stairway, the deck will not need to be entirely rebuilt. A new post and beam system properly placed will solve the issue.
That should prevent a bunch of dancing party goers from unexpectedly cannon balling off the edge of the diving board, deck.