Subsystem sftp internal-sftp
Match Group sftpbackups
        ChrootDirectory %h
        X11Forwarding no
        AllowTcpForwarding no
        ForceCommand internal-sftp

sftp> ls -l
drwxrwxrwx    2 testuser testuser     4096 Sep 25 00:09 backups
sftp> put a.txt backups
Uploading a.txt to /home/testuser/backups/a.txt
a.txt

sftp> ls -l
drwxrwxrwx    2 1290     1291         4096 Sep 30 14:58 backups
sftp> put a.txt backups
Uploading a.txt to /backups/a.txt
remote open("/backups/a.txt"): Permission denied