I like to thank the Ickster for creating this article and for allowing me to post it.

A restructure of a contract doesn't include additional years, an extension
does. If you extend a player an additional year, that does not affect the
spread of his existing bonus. Any new bonus money is spread into the new
year. Usually what happens for an extension is the current year salary is
cut and a signing bonus is paid. That results in a greater commitment to
the player (making him harder to cut and limiting cap flexibility) and more
money pushed into the future.

If a contract has already been restructured, money is converted into bonus
which pushes it into future years. For an extension, you can cut the
salary, but the bonus money cannot then be re-spread over the life of the
new contract. If a contract is heavily leveraged (has been restructured in
the past), that limits how much you can gain by a restructure or extension
in the future because the amount of bonus has increased and that cannot be
spread into future years.

For example, If a player signs a 3 year deal for $12M with a $3M signing
bonus, his bonus money counts for $1M for three years. If, in the 2nd year,
$2M in salary is converted to bonus (pushing $1M into the final year), that
provides $1M in cap relief but raises his bonus allocation to $2M for the
final year. If you then go to extend the player, that $2M in unallocated
bonus from his original contract can't be spread into additional years. His
new bonus (for the extension) can be spread. Thus, his cap number at the
start of the contract is the $2M in bonus from his original contract,
partial allocation of his new bonus and at least minimum salary. You can't
get around the cap hit, regardless of the players age.

Finally, no player with any contract leverage at all is going to accept
adding a year on his deal without a sizeable signing bonus. The only
security a player has in the NFL is signing bonus money. It is the only
thing he is guaranteed to collect, because it is paid up front. For a
player to push the date he is due for free agency, even if it is just a
year, he will want some serious cash up front, if he is a good player.
These guys are mercenaries, they want cash up front.