Gaara surrounds himself entirely in a protective orb of sand to guard against attacks. He then uses the Third Eye to watch his opponents' movements and control sand as necessary to battle them. In addition to being extremely dense, this sand-sphere can form spikes from its surface to spear any would-be attacker who thinks that their own physical strength will be enough to penetrate it. Gaara can also use this technique to shield himself from his opponent long enough to perform his transformation into the One-Tailed Shukaku, as demonstrated in his match against Sasuke Uchiha in the Chunin Exam.
This absolute defense can be broken down by Chidori, as shown when Sasuke versed him in the Chunin Exams, immediately stopping the transformation.