When the wing is fully formed but slows down and starts dropping vertically, it is in Parachutal Stall. Airflow on your face becomes nil and the glider may start to spin.

Power aggravates the possibility as does having an old glider, misrigged lines, high porosity and being light on the glider. Older designs were more prone to parachutal stall than newer designs.

The cure is to immediately go hands up and reduce power. Most pilots won't have time for the next steps which are: tweak the A's (palms forward, twist them down), release the trimmers and step on the speedbar (if installed).

This is quite rare in free flight but, being light on an old glider is still asking for it. That may have been what happened in the video picked below.

It's unfortunately quite common in motoring and is usually a combination of slowing down with brakes, power, torque, turbulence and other factors. Most often it can be attributed to being heavy handed. While it's properly called a spin if a twisting develops but I consider the initiating stall parachutal when the wing does not deform.

Regardless of the definition, the prevention is being very careful when applying brake and the cure is getting off the throttle and reducing brake pressure. If you feel like you're slowing down, reduce power and reduce pressure! And be very leery turning against your motor's torque direction.

Here is a free flight stall so you can see that it does happen, even without a motor.

In the video, you can see by the trailing that he is not pulling any significant brake. Then the wing starts descending vertically and stops flying forward as he swings a bit out in front. The useless brake pull doesn't start until he's about to hit the ground--more a reaction than anything else. Once near the ground, say within 50 feet, it is better NOT to try recovering since the glider will surge forward and you will then free fall the last 30 feet--far worse.