I'm with Tom. A hopback is pre-fermentation, but the wort hits it hot. A Randall is post-fermentation, where the beer hits it cold.
Using a hopback is like adding hops at knockout or in the whirlpool. Using a Randall is like dry hopping. There's nothing special about either method, and there are alternatives. Depends on whether you want heat on the hops or not. The character is different.