Complex sentence: In moderate to medium headwinds, the landings can be without forward speed; or even going backwards with respect to the ground in strong winds, but this would usually mean that the conditions were too strong for that glider.
Split into simpler sentences: In moderate to medium headwinds, the landings can be without forward speed. With strong winds even going backwards with respect to the ground, but this would usually mean that the conditions were too strong for that glider.