Jim asked "Has anybody "step-towed" a sailplane? Using a pay-out winch
in a
truck, tow the length of the runway. As the truck turns around and
winch operator keeps the slack under control, the glider does a wide
turn. The truck then heads back down the runway, paying out more line."

Step launches, or "kiting" has been done by winch and tow car, but not
using the way Jim suggests as far as I know. In strong winds, the
car/truck can just stop, or even track slowly back; the winch can pay
out and then wind in again. It is similar to getting a kite higher
without running, by successively pulling on the string a bit and then
letting it out a lot.

The closest we came to Jim's method at North Weald in the old days of
truck tows was, in a cross wind, tow up the runway and turn into wind
at the end onto the peri track. The glider would have drifted downwind
from the main runway, meanwhile, so could be towed a bit further as the
truck continued into the cross wind. It did not gain much extra,
however - I think the main benefit came from not having to slow down
and stop within the length of the main runway.

Chris N.