fig1 shows a side part 1 of a motor vehicle with a movable front pane 2 and a movable rear pane 3 and , in schematic representation , a control device 4 for operating a front window lifter motor 5 and a rear window lifter motor 6 . in this case , a front window lift button 7 and a rear window lift button 8 are connected to the control device 4 . the front and rear window lifter motors 5 , 6 are furthermore connected to the control device 4 . a rotation speed monitor 9 is connected to the rear window lifter motor 6 and to the control device 4 . when the front or rear window lift button 7 , 8 is actuated , the front or rear window lifter motor 5 , 6 is operated by the control device 4 . in this case , the front pane 2 or rear pane 3 is moved in accordance with a button command . during the movement of the rear pane 3 , the rotation speed monitor 9 records the rotation speed n of the rear window lifter motor 6 , and the control device 4 analyzes the rotation speed n in order to detect a case of trapping . fig2 shows the side part 1 of the motor vehicle with the front pane 2 in the closed state . like the front pane 2 , the rear pane 3 is in a closed position p 0 in the closed state . in the closed position p 0 , a front edge 10 of the rear pane 3 adjoins the rear edge 12 of the front pane 2 by means of a fitted sealing lip 11 . in this case , the front edge 10 and the rear edge 12 are oriented horizontally with respect to the z axis , which is based on the typical vehicle coordinate system . the adjoining position of the edges 10 , 12 prevents relative wind and rain from penetrating into the passenger compartment . fig3 to 6 show the rear pane 3 in various positions as it is moved between the closed position p 0 and an open position p 5 . as the rear pane 3 is moved in the xz plane , a rear pane corner 13 formed at the transition between the rear pane front edge 10 and the rear pane top edge 14 moves along an s - shaped trajectory 15 . in fig3 , the rear pane 3 is shown in a first position p 1 . in this position , the rear pane 3 is tilted slightly in the (−) x direction about a y axis and offset in the (−) z lowering direction relative to the closed position p 0 . in position p 1 , a gap 16 which increases in the (+) z lifting direction is thus formed between the front edge 10 and the rear edge 12 . fig4 shows the rear pane 3 in a second position p 2 , in which the rear pane 3 is at a level approximately half way between the closed position p 0 and the open position p 5 . in position p 2 , the rear pane 3 is tilted slightly in the (+) x direction in the xz plane , and the front edge 10 and the rear edge 12 form a gap 16 which tapers slightly in the (+) z lifting direction but is open . fig5 shows the rear window pane 3 with the rear pane corner 13 in a third position p 3 , in which it has moved along trajectory 15 , with the front edge 10 , which is tilted in the (+) x direction , forming with the rear edge 12 a virtually closed gap 16 which tapers in the (+) z lifting direction and , in the (−) z lowering direction , adjoins a door body top edge or a trim strip 17 located there . fig6 shows the rear pane 3 in a fourth position p 4 , in which the rear pane corner 13 , a shoulder of the front edge 10 and the front pane top edge 14 project beyond the trim strip 17 . in this case , the rear pane corner 13 and the front edge 10 are at a distance from the rear edge 12 . fig7 shows schematically the rear pane 3 in the open position p 5 , in which the rear pane 3 has been moved in the (−) z lowering direction and retracted into the side part 1 . fig8 shows a case of trapping in position p 3 ( in accordance with fig5 ), in which an object 18 is trapped in the gap 16 . the s - shaped movement of the rear pane 3 means that the gap 16 tapers in the (+) z lifting direction in such a way that it is not possible to free the object 18 by movement in a direction in the xz plane . fig9 shows , in a variation diagram , a rotation speed characteristic 19 for the rear window lifter motor 6 in positions p 1 to p 5 as the rear pane 3 is moved in the xz plane . here , a position characteristic shows the position of the rear window corner 13 along the z axis . at an initial time t 0 , the rear pane 3 is in the open position p 5 . in a first interval i 1 , the rear pane 3 is moved in the (+) z lifting direction from position p 5 , via positions p 4 , p 3 , p 2 and p 1 , into the closed position p 0 . owing to the system slack , an abrupt rise in the rotation speed n of the rear window lifter motor 6 at the beginning of the first interval i 1 , with a subsequent rapid drop merging into a normal rotation speed range , is observed . toward the end of the first interval i 1 , the approach to the closed position p 0 is associated with a drop in the rotation speed n . the first interval i 1 ends with the rear window 3 in the closed position p 0 , and the rear window lifter motor 6 stops . in a second interval i 2 , the rear pane 3 remains in the closed position p 0 . in this case , the rotation speed n = 0 . in a third interval i 3 , the rear pane 3 is moved in the (−) z lowering direction . owing to the system slack , the magnitude of the rotation speed n initially rises abruptly but then decreases as the window lifter mechanism between the rear window lifter motor 6 and the rear pane 3 comes into nonpositive engagement . as the rear pane 3 is moved in the (−) z lowering direction , the rear window lifter motor 6 moves in the opposite direction , this being illustrated by a negative rotation speed n . at time t 1 , the case of trapping illustrated in fig8 occurs . in this case , the rotation speed (−) n falls rapidly owing to the trapped object 18 . the control device 4 recognizes the case of trapping from the drop in rotation speed detected by means of the rotation speed monitor 9 and stops the rear window lifter motor 6 . at the same time , the front window lifter motor 5 is switched to a holding state , in which the front pane 2 stops and can no longer be moved . in the case of trapping , the control device 4 switches over the rear window lifter motor 6 immediately in such a way that the rear pane 3 is moved into position p 1 in accordance with fig3 , as illustrated in a fourth interval i 4 . in a fifth interval i 5 , the rear pane 3 remains in position p 1 to enable the trapped object 18 to be freed . position p 1 is advantageous since the gap 16 between the front edge 10 and the rear edge 12 is widened in the (+) z lifting direction . this simplifies the freeing of the trapped object 18 . once the object 18 has been freed , the rear pane 3 is moved into the open position p 5 in a sixth interval i 6 .