Purity: Gradual Immersive Ghusl (Irtimāsī Tadrījī)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 367:
Two types of immersive ghusl can be performed: instantaneous (dafʿī) and gradual (tadrījī).
In gradual immersive ghusl, one must gradually – but in a way that can be commonly considered one single action – immerse his body in water with the intention of ghusl. In this type of ghusl, it is necessary for each part of the body to be out of the water before it is washed.[1]
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
1
An example of gradual immersive ghusl is when a person, with the intention of performing ghusl, immerses part of his body into a bathtub of water and then takes that part out of the water; he then immerses another part of his body and takes it out, and so on until all the parts of his body have been immersed.