Amsteel and tech line are types of cordage. Both are hollow braided so they can be spliced i.e. having the line go into itself allowing for constriction of the the outer over the inner line. This constriction provides the holding power but once its released, the inner line can move freely.
TessdeTerrible is correct. Both UCR's and whoopie slings take advantage of constriction. A whoopie sling is made of one continuous length of amsteel and there is constriction along the full length of the bury whereas the UCR has a separate shorter piece in which the longer line slides through. The constriction of the shorter piece gradually decreases over its length.
Yeah I know, clear as mud.