The Rufsau Rules
Forced play, Davonlaufen (running away), and the special rules around the Sau (ace).
In a Rufspiel you call yourself a partner — the one holding the called Sau (the Rufsau). Four extra rules apply to them, so the Sau doesn’t surface right at the start. Here they are — and at the end you judge for yourself whether you may run away (davonlaufen).
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You must hand over the called Sau
When the called suit (here: Gras) is led for the first time and the Sau has not yet fallen, then as the partner you must give up the called Sau when you follow. It’s just a special case of the Bedienpflicht (the duty to follow suit): the suit is led + you hold the Sau = you must play the Sau. No excuse.
Don’t lead a different Gras
As long as the Sau isn’t yet free, you may not lead any other Gras card than the Sau itself. Meaning: you can’t come out with a small Gras to test the suit — either lead the Sau, or no Gras at all. There’s only one exception: Davonlaufen (running away, just below).
When you may run away
If you hold at least four cards of the effective suit Gras — the Sau among them — AND no Gras has ever been led yet, then you may, as an exception, come out with a different Gras card (not the Sau). That’s called Davonlaufen. From that moment the Sau is free: rules 1, 2 and 4 no longer apply.
Careful when counting: Gras-Ober and Gras-Unter are trump, not Gras — they do not count. Only real Gras (Ass, Zehn, König, Neun, Acht, Sieben) counts.
Don’t schmier the Sau away
When you can’t follow a foreign suit and have to discard, you may not schmier the called Sau onto a foreign trick (schmieren = piling points onto a trick). Only in the 8th (last) trick may you unload the Sau — by then the game is over anyway.
These rules apply only to you as the partner. Everyone else — the declarer included — may lead or follow Gras quite normally, to “search out” the Sau.