Sending cards home
Sending cards to the foundations feels like progress, and sometimes it is a mistake. A card on a tableau column can hold a lower card of the other colour; once it is home, it cannot.
The rule
A card is safe when every card that could still be stacked on it is already home, or is free to follow it there. With both black fives and the four of hearts up, the seven of diamonds is safe: the only cards that could sit on it are the black sixes, and they can leave whenever they like.
What this site does
It sends those cards up for you, so you never have to think about it. Microsoft FreeCell is stricter and holds that seven back until both black sixes are home — both are safe, ours just saves clicks.
Aces and twos
Always safe, always immediately. Nothing can ever need them, and every turn they spend on the tableau is a turn they spend in your way.
On the board, this one reads: Send it home — nothing needs it now that 6♣ and 6♠ can follow.