The spoon (כף) ben yomo and assessing sixty against what was dipped, נותן טעם בר נותן טעם of heter, the knife (סכין) that cuts hot meat, קליפה, הגעלה and נעיצה (Shulchan Aruch, Yoreh De'ah 94 — 9 se'ifim)
הַתּוֹחֵב כַּף חוֹלֶבֶת בִּקְדֵרָה שֶׁל בָּשָׂר, אוֹ אִיפְּכָא — מְשַׁעֲרִים בְּכָל מַה שֶּׁנִּתְחַב מִמֶּנּוּ בַּקְּדֵרָה, אִם הַכַּף בֶּן יוֹמוֹ, דְּהַיְינוּ שֶׁשִּׁמְּשׁוּ בּוֹ בִּכְלִי רִאשׁוֹן תּוֹךְ מֵעֵת לְעֵת.
One who dips a dairy spoon into a meat pot — or the reverse — one assesses (the sixty) against all of it that was dipped into the pot, provided the spoon is ben yomo, that is, that it was used in a kli rishon within twenty-four hours.
Shulchan Aruch, Yoreh De'ah 94:1LEVEL 01
Hebrew text of the 9 se'ifim with a fluent English translation. The spoon (כף) ben yomo and assessing against what was dipped, נותן טעם בר נותן טעם of heter, the dairy knife (סכין), קליפה, הגעלה and נעיצה explained with practical cases.
📖 StudyLEVEL 02
In-depth pilpul: why one assesses against what was dipped (לא ידעינן כמה נפיק מיניה, Shach sk1), the debate of חם מקצתו חם כולו (Taz sk12, Rash"al on the whole blade), the yesod of נותן טעם בר נותן טעם דהיתרא, the knife ben yomo / not ben yomo, חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (spoon ben yomo or not, with / without sixty, kli rishon / kli sheni, knife ben yomo or not), golden rules, classic pitfalls (נ\"ט בר נ\"ט, חם כולו, קליפה, נעיצה) and memorization of the 9 se'ifim.
📖 StudyLEVEL 04
The practical halacha according to the Shach, Taz, Pri Megadim and Pitchei Teshuva (Noda BiYhuda, Chatam Sofer), then the Sephardic poskim (Yabia Omer, Yalkut Yosef, Or LeTzion) and Ashkenazi poskim. Note: the Shulchan Aruch HaRav does not deal with this siman — this is a level of psak, not "Daat HaRav".
📖 StudyNot necessarily (YD 94:1, 3). If the spoon is ben yomo (used in a kli rishon within 24 h), one assesses sixty against the part of the spoon that was dipped into the pot (לשער נגד מה שנתחב); if there is sixty, the pot and the dish are permitted, but the spoon itself remains forbidden since it is absorbed with basar be-halav. If there is no sixty, everything is forbidden, even for benefit (בהנאה), including the pot — yet one may still place fruit or cold items in it. For practical halacha, consult your Rav.
This is the principle of se'ifim 5 and 9: a permitted taste (דהיתרא) that passed through a neutral intermediary — water, a pot, a utensil — before reaching the other kind does not forbid bedieved. Thus honey cooked in a meat frying-pan bat yoma and then poured hot into a dairy bowl bat yoma is permitted, for this is נותן טעם בר נותן טעם דהיתרא. Likewise the new pot in which two spoons were dipped: the custom is to forbid its use lechatchila, but bedieved everything is permitted. This is distinct from נ"ט בר נ"ט when actual prohibition (בעין) remains (cf. siman 95). For practical halacha, consult your Rav.
It depends (se'if 7). Boiling meat in a kli rishon cut with a dairy knife that is ben yomo: the entire slice is forbidden if there is no sixty against the spot of the knife, and the knife itself requires הגעלה. If the knife is not ben yomo (or it is unknown), it forbids only כדי קליפה (the thickness of a peel), because of the grease (שמנונית) of the knife. In a kli sheni, the meat requires קליפה and the knife נעיצה (thrusting it into the ground). For practical halacha, consult your Rav.