Clipnodes
Clipnodes are the method BSP29, BSP30, and some derivatives use for collision on hulls 1 and greater.
In VHLT, the KV zhlt_noclip
can be used to disable generation of clipnodes for any brush entity (including func_detail and func_group), leaving only hull 0 (the point hull, which also is the rendered mesh).
BSP38 and derivatives such as Source 1 BSPs use brushsides instead.