diff --git a/bsi.go b/bsi.go
index 308de8e0b..7d76ec3c8 100644
--- a/bsi.go
+++ b/bsi.go
@@ -24,9 +24,9 @@ import (
 type bsiData []*Row
 
 // pivotDescending loops over nonzero BSI values in descending order.
-// For each value, the provided function is called with the value and
-// a slice of the associated columns. TODO, document branch and
-// limit/offset behavior (particularly when nil).
+// For each value, the provided function is called with the value and a slice of the associated columns.
+// If limit or offset are not-nil, they will be applied.
+// Applying a limit or offset may modify the pointed-to value.
 func (bsi bsiData) pivotDescending(filter *Row, branch uint64, limit, offset *uint64, fn func(uint64, ...uint64)) {
 	// This "pivot" algorithm works by treating the BSI data as a tree.
 	// Each branch of this tree corresponds to a power-of-2-sized range of BSI values.