Technique
Top up
To top up is to fill the rest of the glass with a mixer such as soda, tonic or sparkling wine after the other ingredients are in.
In practice
Pour the mixer in last and slowly, down the side of the glass, then give it one gentle stir so it keeps its bubbles. Recipes say "top up" instead of an amount because it depends on the size of your glass and how much ice is in it.
Drinks to try it with
Top up, in short
What does top up mean in cocktails?
To top up is to fill the rest of the glass with a mixer such as soda, tonic or sparkling wine after the other ingredients are in.