Normally when you use the image window controls to move through the front or back clip, each frame of the clip is loaded and appears in sequence in the image window. You can change this default setting so that the frame displayed is not updated as you move through the clip, but remains locked at a specific frame.
To lock the front or matte clip:
You lock the back clip in the same way as the front clip, except that you select Lock from the Back Clip box.
The matte clip is locked with the front clip by default. However, you can slip the matte so that it is locked at a different frame. Use the Slip field to set the slip value for the selected matte clip.