Liquid fabric softeners (eg Fluffy) contain cationic surfactants which are positively charged, they are attracted to the negatively charged fabric, coating the surface, causing threads to ‘stand up’ making the fabric feel softer. The conditioning component of the surfactant add lubrication.
These fabric softeners can be used but infrequently, for example every third wash. A mainstream detergent and hot wash is needed to ensure any softener residue is removed in the subsequent wash,and does not build up reducing absorbency.
2 in 1 detergents that contain fabric softener (eg Cold Power + Cuddly, Biozet + Softener, Surf 2 in 1) work differently. The fibers repel each other which causes everything to stand up and become fluffy. They don’t reduce absorption. These types of detergents can be used continually without any issue.
See Laundry Detergent Ingredients WashWise for more information.