The ‘Bet view slider’ helps you to see the odds available for the amount you want to bet.
When you log in you will see your default bet view value, which has initially been set to £2 (€2). You can change this default value in the ‘Display Options’ . You can also move the slider on any market you look at.
Let's say you can see this:
You want to bet £40 so you slide the view to £40 to see what odds you can take now:
You can see that £55 is available at odds of 5.2 or better.
Betfair always matches your bet at the best odds available (called ‘Best Execution’), so in this example if you place £40 at 5.2 we'll match £12 of it at odds of 5.4 and the remainder at 5.2.