Birkas Hamazon cancels the berachos that were said on food and drink from before. Therefore, a new Hagoffen is required when drinking the kos shel beracha after bentching. (ShA1 – MB1)
It is necessary to drink at least a cheek full (most of a r’viis) from a kos shel beracha. That amount puts you into an argument among the poskim regarding the need for a beracha achrona. You should therefore drink a complete r’viis so you can say the beracha achrona according to all opinions. (ShA3)
For all drinks you should be careful not to drink an amount from a k’zayis to a r’viis to avoid the doubt regarding a beracha achrona. (MB9)
Say an after beracha without delay after drinking. Otherwise, the short time it takes to begin digesting it may pass, and you will lose the mitzva of bentching after the drink. (MB8)
The sages didn’t make different shiurim for different drinks; therefore, no beracha achrona is required even after a shot of whiskey unless it was a r’viis. (MB14)
Someone must drink a cheek full from the kos shel beracha, so if the mevoraych doesn’t want to drink, he can say the Hagoffen and be motzi any volunteer who ate at the meal. The cheek full amount cannot be shared by two people. (ShA4 –MB15,16)
If there is enough wine in the kos for everyone else to taste some after the mevoraych drinks a r’viis, they have a mitzva to do so. (ShA4 – MB17)
When the kos is passed around for all to sip, you will need to bless Hagoffen unless you fulfil all the following conditions:
You had specific intention to be yotzay with the mevoraych’s beracha knowing there might not be any left.
He had you in mind,
You didn’t talk in between his beracha and your tasting. (MB19)