The Bachmann hitch is a slide-and-grip knot used to ascend a rope. The Bachmann hitch is tied with a loop of accessory cord or paracord.
The carabiner helps loosen the hitch when sliding up the rope.
When ascending, you might be tempted to pull down on the carabiner to help you stand up. If you do this, the Bachman hitch will loosen and not hold your weight.