Isosceles Triangle (Quick Construction Technique) V1

The applet below illustrates a quick (and easy) way to construct an [b][color=#38761d]isosceles triangle[/color][/b] using a [b]compass and straightedge.[/b] [br][br][br][b][color=#0000ff]Can you explain why is this method of construction valid? [/color][/b]

Information: Isosceles Triangle (Quick Construction Technique) V1