From the manual:
Drag Copying
We’ve mentioned the Cut, Copy and Paste functions above, along with their keystroke equivalents. SpeedEDIT offers another great copying convenience: if you hold down the Ctrl key before you start dragging a clip or selection of timeline items, you make a copy of the object(s) at the location where you release the mouse button. The source items remain in place.
This is a great way to quickly copy a clip … unless, of course, you just wanted to move it -- if you do this by mistake you’ll leave unwanted copies in the wake of your mouse. Remember the earlier hint (from Section 2.2 Cut Copy, Paste) regarding the usefulness of multi-selection copy/paste operations when combined with Inherit – this is a dynamite way to quickly flesh out project ‘templates’ for regular use.