Tell tale shot ...

I know that it is recommended to add a 'tell tale' shot just above the hook when float fishing. What is the effect of this and how does it work?

TIA

M

It tends to give the indication of a lift bite.

It works better if your float is at least two different colours.
For arguments sake, let's say the last half inch of your float is red, and the rest is black. Dot the float down so that where the red and the black join, it is just above water level.
When you put the tell tale shot on, the float should sink so that only the red is showing.
If the black starts to show, that is an indication of a lift bite.

I used to have a white band in between the red and the black, so I could more easily spot a bite no matter which way the float went. (up or down)