Right, it's just Galilean relativity. The vertical version of wind
drift or wind correction angle between heading and track. The glider is
always descending through the surrounding air, but if that air is rising
faster than that rate relative to the ground, there is a net gain.

Many airplane pilots find it puzzling that a glide attitude is
maintained while climbing. Also seemingly paradoxical is that spiraling
at a steep angle of bank often results in a better climb rate in a
thermal. Yes, the sink rate of the glider is higher at the higher bank
angle, load factor, and airspeed, but the vertical speed gradient of the
air (faster in the "core") can more than overcome that.

--JHC