To respond to a couple of the commentators:
"at what speed did you do the sideslip?" - In all probability the answer will be that the ppilot doesn't know because there is so much disturbance to the airflow that the ASI will read completely incorrectly
"What speed was the glider trimmed for" - Unlikely that trim position will have much bearing (except that if it's trimmed forward this may give you slightly more elevator authority)
Spoilers - In the UK most gliders have airbrakes (which generate drag)
"modern glass ships generally have such powerful spoilers that you don't need to bother with sideslips" - Believe me, if you're going into a small field in a slippery glider then you may need anything that helps decrease the distance to travel to lose height when trying to squeeze into a field that you're at risk of overshooting
"In many you can simply pull full spoilers and point it at the chosen landing spot" - most airbrakes are speed limiting only up to about a 40 degree downline. Beyond that the glider may continue building up speed.
Homeguard looks like a good summary of how it works.