This soft release deal seems to be a lot more common than I thought!
Searching the Internet brings up some interesting stuff. Here is
something form a SOP manual for a glider club:

Soft Release Procedures
A soft release, or low-tension release, is standard operating
procedure at . We do this to prevent damage to the fuselage structure
and excess wear of the Schweizer tow hook which can occur after
repeated high-tension releases. A soft release is performed in all of
our aircraft for standardization. As an added bonus this type of
procedure also all but eliminates harmful knots forming in the towline
after release and alerts the tow pilot that you are about to release.
The purpose is to relieve some of the tension on the towline, not put
a lot of slack in the line. When a “bow” begins to form in the
towline, the tension has been reduced enough to release.

To perform a soft release:
1) HOLD the release handle with your left hand.
2) REPOSITION slowly to just beyond the right wing of the towplane.
This is done to enable the glider pilot to continuously view the
towplane during the following steps, and ensure rapid and visible
separation from the towline after release.
3) Initiate a BRISK CLIMB until the towplane appears about 10 degrees
below the horizon.
This climb must be rapid enough to accelerate the glider. This speed
difference relative to the towplane is what will create the slack you
want.
4) PITCH OVER to stop the climb BUT NOT ENOUGH TO DECEND.
5) Once a “bow” forms in the towline forms, RELEASE.
At this time the glider should be level with the towplane or slightly
above. With proper slack in the line you should here little to zero
noise when releasing the rope.
6) After release, TURN RIGHT to clear the towplane and rope.
*** Never dive to accelerate the glider. This is not as effective as
climbing and may cause you to fly below the towplane