In hang gliding a sprog is an internal strut used to control sail twist on "topless" gliders instead of the reflex bridles used for that purpose on kingposted models. The first wave of topless hang gliders appeared at the pre-world championships in Australia in 1996 (the worlds are held every two years, with pre-worlds held at the site of the next worlds on off years). The gathered pilots from around the world were all examining and talking about the innovations on their wings, and somehow the term "sprog" came to be the term of choice for the internal struts. Through reportage about the competition the application of the term quickly spread and has stuck. The term "sprog" was later said to be Aussie slang for sperm, though a little research only shows it as referring to offspring (affectionately) or, in the military, to recruits or any neophyte.

A hang glider sail (wing) is flexible by design, with the trailing edge able to move somewhat relative to the leading edge. To help ensure that a glider will recover from a very steep dive without pilot input, some means of supporting the trailing edge at very low angles of attack is needed, traditionally accomplished with reflex bridles - cables running from the top of the kingpost (a few feet above the wing) to the trailing edge of the sail. Topless gliders, developed in the mid-1990s, did away with the kingpost and upper rigging to eliminate a major source of performance-robbing parasitic drag.

Without the kingpost, there was nothing upon which to hang reflex bridles, so some other means of trailing edge support was needed. The most obvious solution was a strut inside the sail, anchored at the back of the leading edge frame tube and braced with a cable running from the top of the trailing edge tube to some point in the middle of the strut. Reflex bridles typically run to 3 or 4 points on the back of each wing, at the rear of the mid-span battens; one topless glider had 4 of the internal struts per wing (all light but expensive carbon composite constructions). Most models used one or two struts per side, each of which supported a spanwise rod near the trailing edge (transverse batten) which supported two or three battens.

The design problems were that the strut needed to be light, pivot or detach to allow the glider to fold normally, and support a couple of hundred pounds - the last entailing a couple of thousand pounds of tension on the bracing cable and hardware, plus a corresponding compression load on the front pivot point. A bracket with a universal joint was fairly straight-forward exercise, and wieght was addressed with carbon composite reinforced aluminium tubes at first (later larger diameter aluminum tubes without carbon were used). The bracing cables required more exotic fittings than usual but were otherwise unremarkable. The transverse battens also had to be stiff and light, so composites were used again, both carbon-balsa sandwich structures and simple carbon tubes.

These internal struts weigh much more and cost more to make than a kingpost and reflex bridles, but the benefit of drag reduction is significant. It is important that the sprogs not raise the trailing edge during normal flight, so there is a means of adjusting the height of each sprog. Lowering the sprogs reduces drag and pitch forces at high speed, but also reduces pitch stability, which is dangerous. Many of the competition pilots fly with their sprogs lowered to the point that their gliders would not pass certification standards, though very very few have tumbled or gone into unrecoverable dives.

A few gliders later included compensator systems, with the sprogs linked to the vg systems, so that the sprogs were higher at lower VG settings, and thus theoretically more effective, but the value of these systems is a debated question.
Update, March 2009
Sprogs have come to be used on kingposted gliders, both 'advanced intermediate' gliders and intermediate gliders. The sprogs in these cases allow elimination or reduction of the number of reflex bridles needed for stability, with a performance gain from the reduction in parasitic drag.

Sprogs made from carbon composite tubing are common advanced options for the topless gliders, as resilient ways of attaching the cables to the carbon tubes have been worked out. Early on there were problems with the attachment hardware tearing through the carbon composite under heavy load, meaning the sprogs would have to be replaced after a serious glider pitch attitude incident (though they would work that one time). The lighter carbon composite sprogs provide the benefit of reduced inertia in roll.

Less flexible 1x19 cables are used to brace sprogs in some gliders where 7x7 cables were used in the past; because the 1x19 cables stretch less under load they can be set slightly lower.

Many competition pilots flying in cross country races lower their sprogs, because the reduced twist of the wing gives them a better glider ratio at higher speeds. Some have as a result had their gliders tumble in turbulence. There is a move on within the CIVL, which governs international competitions, to determine minimum sprog heights for competition gliders and to implement standards and measurements in sanctioned competitions, as a safety issue. This is a notion fraught with problems that will likely cause more problems than it solves. There are a number of glider tuning issues that play into overall stability, many variables, many different gliders, changes in design over time, etc.