Glider pilots are taught to look over the nose most of the time when turning so as to maintain the correct attitude (and hence airspeed). When turning final there is a tendency to look at the landing area instead. This frequently results in the nose rising a bit and airspeed consequently being reduced. Many gliders tend to change attitude when the airbrakes are opened - with some the nose drops and others the opposite. Superimposing that effect on a possible wrong attitude due to the pilot looking at the landing area can lead to a dangerous loss of airspeed.
To mitigate the risk pilots are advised to open air brakes preferably after the final turn is complete and when on a 2/3 path as mentioned in the earlier post or, if excessively high, before the final turn - but not during the turn. They are also encouraged to do a well banked turn (say 30 degrees) since there is evidence that it is more difficult to spin from such a turn as compared to a flat turn if speed is lost.