The major unmount achievement to be unlocked is the read-only remount. Then any new accesses to filenames in the below the mountpoint will hit the newly overlaid directory with zero permissions - new blockers to the unmount are thereby prevented. This same behaviour can be achieved by mounting an empty directory with permissions 000 over the directory to be unmounted. The useful behaviour of umount -l is hiding the filesystem from access by absolute pathnames, thereby minimising further moutpoint usage.
It can cause btrfs filesystem corruption.It doesn't actually unmount the device, it just removes the filesystem from the namespace.Umount -l is dangerous or at best unsafe.
At the time of writing, the top-voted answer recommends using umount -l.
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