When you first start flying gliders one of the things you are shown then soon made to practice for your check-ride is Boxing the Wake. This is a maneuver you do on aerotow where you first go down through the wake of the tow plane to a low tow position (under the wake).
You typically start in the low tow position then progress to either lower left or right corner. Then the upper corresponding corner. Back to the low tow position and back up through the wake to the normal high tow position.
The wake is caused by the prop of the plane disturbing the air it is pulling the plane through. When you go through the wake for the first time early in lessons it can be unnerving feeling. But it is not dangerous and with practice Boxing the Wake can be fun since you are able to practice stick and rudder skills to do it proficiently.