the student has left the flare a little late so you are low and fast, and you (the instructor) didn't catch it in time. Fast slide-on with a collective pull will take care of it, let the skid gear do its job.
If you're not in a position to accept a fast ground slide, this will also work as a last-ditch bum-saver:
Student leaves the flare too late (which is kind of your fault anyhow, but having got yourself in the position you have to deal with it);
Take over, reef up on the collective to quickly get some height under your tail, then immedately flare hard and lower the collective again to regain / maintain rotor rpm (flare 'zoom' and collective reduction should balance eachother out if judged right, keeping you at the right height);
Now you should be back in a relatively normal 'end of auto flare' position, continue as usual.
It's not for the faint-hearted, but it does work.