Vaisala RS41 - Manual (Page 27)

Chapter 3 ________________________________________________________________ Operation
VAISALA_______________________________________________________________________ 25
If necessary, push the unwinder stick in two places: hold the radiosonde
in your left hand and push the stick with your thumb as shown in
Figure
9 on page 24 . Use your right hand thumb to push the stem of the stick
against the edge of the radiosonde.
Make sure that the unwinder string does not unwind too much at this
stage. It might get tangled during the preparations.