Contingent upon file codec/key frames, instead of using the slider bar, I find it easier to fast forward by "scene cuts" (far right button in the red box) WHILE repeatedly bumping the "right arrow key frame" advance button during segments you wish to keep. You can zip through the file quickly and left key frame bump once you have gone too far:
Scene cut is a scan "toggle" (on/off) in either direction, until the next scene cut; key frame is a single "bump" in either direction to the next closest key frame.
If you experiment with these 4 buttons while your video is loaded then it will make better sense. Click a button, wait, observe what happens to the time index. Do the reverse. Observe. Use the other buttons. Observe.
(More advanced: Scene cut threshold sensitivity can be adjusted.)
With a traditional avi, and proper capture, you are best off cutting at (K)ey frames (not I or B) anyway.
If you find that the key frame interval is vast then you should probably GENTLY DECREASE the key frame interval in your recording software (codec), which increases the number of key frames.
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("vdub mod", also freeware, supports other "more tricky" typically "unsupported" formats, but you will likely not need this unless you work with DVDs, etc)