Cass wanted to train but set the holographic training room to a random opponent and it gave her the original Batgirl (who is Oracle, and in the next room and can watch what's going on with her monitors).
Cass is like "okay, cool!" excited to see her mentor and superheroic namesake as she was in back in the day, but the fight keeps ending really quickly, and Cass can't understand how Babs could have survived if she was so easy to take down. But Cass kept replaying the program before holographic!Babs could follow through with her next move, which was catching the oblivious and unsuspecting Cass around the ankles.