the device of the invention and thus on the user 39 s control over the tension force in the cable an advantageous aspect of the invention also consists in monitoring the angular position between the two moving parts 20 and 30 to do this marker means may be provided the marker means may operate visually thus as shown in fig1 the first moving part 20 has a plurality of patterns disposed on a circular arc substantially centered on the transverse pin 50 these patterns are masked by the second moving part 30 with the exception of one pattern that can be seen through an opening 36 a provided for this purpose in the second moving part 30 when the device is actuated the moving parts pivot relative to each other so the opening 36 a of the second moving part 30 shifts relative to the first moving part 20 thus allowing a new pattern on said first moving part 20 to appear consequently the pattern that is visible is a function of the angular position between the two moving parts it is possible to use a green spot when the motor and gearbox unit is released and a red spot when.