Bererah is an option to designate after the fact a specific item between two equally possible items. For example:
If a man buys wine from among the Cutheans (who did not tithe their wine) he may say: ‘Two log which I am about to set aside are terumah, ten are first tithe and nine are second tithe’, and this he redeems and may drink [the wine] forthwith
The specific two log of wine are set aside later. Normally this would have to be done before the purchase was complete – but here, the wine is being purchased at dusk before Shabbat begins. The wine is necessary for Shabbat, but there is no time for separation.
Now this was not popular and many of the rabbis disagreed with its viability:
They said to R. Meir, ‘Do you not agree that the wineskin might burst and the man would thus have been drinking liquids of [forbidden]tebel?’
You may hope to designate it later – but what if the wine is spilt and ruined?
And he replied: ‘When it will have burst [there would be time for the question to be considered]’
And when is the time for that consideration? Ex post facto!