diff --git a/.murdock b/.murdock
index 98902ccb8a..c664af33af 100755
--- a/.murdock
+++ b/.murdock
@@ -178,10 +178,18 @@ check_label() {
     return $?
 }
 
+# this function returns 0 when this is build is building one of the two bors
+# branches ("staging" or "trying", 1 otherwise.
 is_bors_build() {
     test "${CI_BUILD_BRANCH}" = "staging" || test "${CI_BUILD_BRANCH}" = "trying"
 }
 
+# Set CI_BASE_COMMIT in case of a bors build.
+# This assumes that bors always adds a single merge commit (possibly itself
+# including multiple merges for PRs) on the base branch.
+# That git command outputs the hash of the second merge commit (the one before
+# the topmost merge), which should be the base branch HEAD.
+# (CI_BASE_COMMIT is used by `can_fast_ci_run.py` to only build changes.)
 if test -z "${CI_BASE_COMMIT}" && is_bors_build ; then
     previous_merge="$(git log --merges --max-count 1 --skip 1 --pretty=format:%H)"
     export CI_BASE_COMMIT="${previous_merge}"