By simple definition, an improvised weapon is a found object used as a weapon. In this scenario, the attacker’s shirt was used against him to choke him out unconscious.
I strongly advise the readers to seek proper instruction from a qualified self-defense instructor in learning chokes or sleeper holds. Sleeper holds result to unconsciousness by applying pressure on the carotid arteries on the sides of the neck temporarily cutting the supply of oxygenated blood to the brain. In practice, the practitioner must immediately let go of the hold or stop applying pressure as soon as his training partner limps and loses consciousness. Needless to say a qualified instructor must know proper resuscitation techniques. Incorrect application of a sleeper hold could result to serious physical injury or even death. So be careful when practicing this technique.