Schmieren
You can't always win a trick yourself, but you can still make it worth more. As a defender you drop a card full of Augen onto a trick your partner is already winning: those Augen count for your side even though you never took the trick yourself. The Skat word for it is Schmieren.
Read the official rule on what each card is worth in Augen.
Partner leads the Club Jack, and nothing beats it. Your Ace of Hearts has nowhere better to go.
When it's safe
The trick is safe as soon as nothing still outstanding can beat your partner's card. If they lead the Club Jack, the highest trump in the round, it's settled before the declarer has even played a card. You're just as safe when you play last and have already watched the declarer follow suit below your partner's card. You only get a free choice once you're out of the suit that was led, though: a trump lead has to be answered with trump while you still hold one. Once you are out of it, add your most valuable spare, the Ace of Hearts for eleven Augen or a Ten for ten, without a second thought.
When it's risky
If your partner has led and the declarer only plays after you, nothing is settled yet. Say your partner leads the King of Spades and you're out of that suit. Throw the Ten of Hearts, a card full of Augen, and if the declarer still holds the Ace of Spades or ruffs in with trump, your ten Augen land on their pile instead of yours. Throw the Seven of Diamonds instead, and hold your Augen back until the trick is genuinely decided.
Playing for Schneider
Augen only count once they're banked in a won trick, and at the Schneider line it often comes down to a single card. As defenders you need 31 Augen to escape Schneider; the declarer needs 90 to hold you under it. Say it's the last trick and you're on 18 Augen, your partner leads the Ace of Spades, the declarer follows with the Seven of Spades, and you're out of that suit. Add the Ten of Hearts and you're on 39, with Schneider off the table. Throw the Seven of Diamonds and you stay on 29, and the declarer scores a multiplier level higher.