RadSlider provides an increase and decrease handles, which provide an alternative way of moving the thumb along the track. To enable the handles you have to set the HandlesVisibility property to Visible.
<telerik:RadSlider HandlesVisibility="Visible" />
In case you enable the handles when RadSlider is in selection range mode, then by default clicking for example the increase(right) handle will increase the value of SelectionEnd, while clicking the decrease(left) handle will decrease the value of SelectionStart.
<telerik:RadSlider x:Name="slider" HandlesVisibility="Visible" IsSelectionRangeEnabled="True" Maximum="15" Minimum="0" SelectionEnd="8" SelectionStart="5" />
If you want to control the behavior of the RadSlider's Handles when the SelectionRange is enabled, you can take advantage of the StepAction property. The property is enumeration of type StepAction and exposes the following members:
ChangeRange - clicking the handles will increase the SelectionEnd or decrease the SelectionStart of the selection range. This is the default behavior of the control.
MoveRange - clicking the handles will move the selection