Dad buys stock at $200K.
It grows to $10M.
If he sells, he owes tax on a $9.8M gain.
So he does not sell.
If he needs cash along the way, he can borrow against it instead of selling. The loan is not taxed.
But mostly he just holds.
He dies still holding the shares.
The basis resets to $10M the day he passes.
The kids inherit at $10M.
That $9.8M gain. The income tax on it disappears.
They can sell the next day and owe almost nothing.