What Qualifies
All five cards must be 9s or 10s. Suits do not matter, and any single card above a 10 ends it: an ace, a King, a Queen, or a Jack, including a Jack that would have become a bower once trump was named.
The cards are all discarded, the same dealer shuffles and deals again, and bidding restarts from the dealer's left. Nothing is scored and the cards never come into play. If the fresh deal produces another qualifying hand it goes again, but only so far: after three redeals the next hand stands however weak it is.
Why the Hand Is Unplayable
Your best card is a 10, and 10 beats exactly one thing: the 9 of the same suit. Name any suit trump and your trump holding is the bottom of it. Defend, and nothing you hold takes a trick off anyone who called. There is no trump choice, no timing, and no partner good enough to get a trick out of it.
That is the difference between this rule and Ace No Face, and it is why this one draws less argument. An ace with four low cards is a real trick if you time it. A farmer's hand is not a weak hand, it is no hand, and your partner spends five tricks effectively alone against two, with none of the loner bonus that would normally come with playing a hand by yourself.
The case against the rule is that taking bad hands is part of the game, and that learning to defend from a dead seat teaches you something. It is a fair argument. It is also five tricks of watching.
How Often It Happens
About one deal in 759 for any particular player, which makes it the rarest of the redeal triggers by some distance. Most regular players have seen one. Nobody sees them often.
How It Works on detroit.games
Off by default. It is a toggle in the game settings when you create a game, and the house-rules Just Deal button turns it on alongside Ace No Face. Games waiting for players carry a "Farmer's Hand" badge in the lobby list and detail panel, so you can see it before you sit down.
There is nothing to declare and nothing to take on trust, which at a live table is the rule's awkward part: either you expose the hand and leak information, or the others take your word for it. Here the server checks every deal as it happens. The qualifying hand is turned face up for the whole table for 10 seconds, then the new deal starts.
It works the same way in three-handed Cutthroat, where every hand is checked exactly as it is at four.
See Also
The euchre rules page covers the deal this interrupts. Ace No Face is the milder version of the same idea, and Stick the Dealer is the house rule that runs the other way, forcing a hand to be played rather than thrown in.