אורח חיים קע״טOrach Chayim 179

Se’if 1א׳

גָּמַר סְעוּדָתוֹ וְנָטַל יָדָיו מַיִם אַחֲרוֹנִים, אֵינוֹ יָכוֹל לֶאֱכֹל וְלֹא לִשְׁתּוֹת, עַד שֶׁיְּבָרֵךְ בִּרְכַּת הַמָּזוֹן; וְאִם אָמַר: הַב לָן וּנְבָרֵךְ, הָוֵי הֶסַח הַדַּעַת וְאָסוּר לוֹ לִשְׁתּוֹת אֶלָּא אִם כֵּן יְבָרֵךְ עָלָיו תְּחִלָּה, וַאֲכִילָה דִּינָהּ כִּשְׁתִיָּה לְהָרֹא"שׁ; אֲבָל לְהר"ר יוֹנָה וְהָרַ"ן אֲכִילָה שָׁאנֵי, שֶׁאַף עַל פִּי שֶׁסִלֵּק יָדוֹ מִלֶּאֱכֹל וַאֲפִלּוּ סִלְּקוּ הַשֻּׁלְחָן אִם רָצָה לַחֲזֹר לַאֲכִילָתוֹ אֵינוֹ צָרִיךְ לְבָרֵךְ פַּעַם אַחֶרֶת, שֶׁכָּל שֶׁלֹּא נָטַל יָדָיו לֹא נִסְתַּלֵּק לְגַמְרֵי מֵאֲכִילָה.

Which things are considered a lapse of attention during a meal requiring a new blessing. 6 sections: If one finished his meal and washed his hands with the final waters, he may not eat or drink until he recites Grace After Meals. If he said "let us say the blessing" this constitutes a lapse of attention and he may not drink unless he first recites a blessing over it. Eating has the same law as drinking according to the Rosh; but according to Rabbeinu Yonah and the Ran, eating is different — even though he withdrew from eating, and even if the table was removed, if he wishes to return to eating he need not recite a blessing again, since as long as he has not washed his final waters he has not fully withdrawn from eating.

Se’if 2ב׳

מִי שֶׁסוֹמֵךְ עַל שֻׁלְחַן אֲחֵרִים, אֲפִלּוּ אָמַר: הַב לָן וּנְבָרֵךְ, לָא הָוֵי הֶסַח הַדַּעַת עַד שֶׁיֹּאמַר בַּעַל הַבַּיִת.

One who depends on another's table — even if he said "let us say the blessing" it is not considered a lapse of attention until the host says so.

Se’if 3ג׳

אִם לֹא אָמַר הַב לָן וּנְבָרֵךְ וְגַם לֹא נָטַל יָדָיו מִשֶּׁנָּטַל הַכּוֹס לְבָרֵךְ, הָוֵי הֶסַח הַדַּעַת.

If he did not say "let us say the blessing" and also did not wash his final waters — once he has taken the cup to recite the blessing it is considered a lapse of attention.