BHAV Resources contain the code for object behavior.
IFFSnooper displays this code as a series of lines of hex data with text interpretation.
BHAV Resources can be viewed in the Hex Data window by selecting Resource > BHAV from the Resource pop-up menu.
BHAV Resources are editable via the Hex Data window as of version 1.2.3
View of a BHAV resource in IFFSnooper 1.2.4.
BHAV resource editing can get quite complicated but there are some introductory notes here.
More resource information