Move start date left
Click 
to move the start dates of the selected tasks or links to the left within the bar chart, ie to an earlier date. The start dates of the selected tasks or links move left by one of the current snapping time units each time you click the control. For example, if the current snapping time unit is weeks, the start dates of the selected tasks or links move a week earlier.
The finish dates of the selected tasks are not changed, so the tasks increase in duration by one of the snapping time units each time you click the control.