With all respect, that's a very tricky game to play. Yes, you can apply a lot more shock force by "taking a run at it." But it substantially increases the possibility of a) the tow line breaking and whipping back, b) one of the vehicle/tow line connections failing under shock load, or c) damage to the vehicle(s) themselves.

The best and safest course is always to take up the slack on the tow line, whatever kind it may be, and then apply force. For stretchy lines, this will provide plenty of "spring." For low-stretch lines like cable or chain, a slight amount of sag will provide some of that spring (better combined with a short stretchy piece if possible).

In any case, IME, if you have more than a few inches of slack in your tow line, you're asking for trouble.

EDIT: You guys beat me to it in the last few posts. I need to refresh the thread before I post anything!