How to decrease trackpad tracking speed on demand?
BetterTouchTool is the free tool of your choice:

With a simple modifier key you may decrease or increase tracking speed temporarily after configuring the different speeds:



To toggle cursor speed, create a new gesture (optional with a modifier key) or a keyboard shortcut (no screenshot here - just choose the keyboard menu instead of the trackpad menu) and choose the predefined action -> Other Mouse Actions -> Toggle Mouse Speed. 



I don't own a Magic Trackpad but this works at least with my MacBook Pro trackpad.