Dynamic time distortion over a single clip (noob question)

Hi.

I know there has to be a tutorial on this somewhere but I'm such a noob I don't even know what keywords to search on to find what I need.

The specific effect I am trying for:

I have a timelapse running at 16,000% speed and I want to slow down to about 20% speed to highlight a lightning strike caught in the footage, then go back up to 16,000%

I'm pretty sure that Time Remapping should be able to do this, and I have even watched a tutorial on it, but I don't seem to be able to make it work. Whenever I try to adjust the 'rubber band' the opacity gets adjusted instead, regardless of which effect or bezier handle I think I have selected. I've tried fiddling with controls but really I'm fumbling in the dark.

Pointers to a guide / tutorial would be very appreciated.

Mitch