John,
Middling is taking both sides (or totals) at different shops with different speads.
ie.
Spurs -4.5 -104 Pinnacle
Nets +5 -107 Canbet
If the Spurs win by 5 you win won side and push the other ... in the neighbor of a 20-1 payout if you hit (approx - it depends on the juice).
Hedging,
If making another risk that will either minimize your loss or guarantee you profit on another bet.
Say you took Jersey (i don't know the real odds) at 6-1 at the start of the Playoffs to win the series ...
.. they once they get to the final you took the Spurs -200 to win the series ...
if you bet 100 on NJ early, and $400 on the Spurs your guaranteed to net $200 for your early success on the Nets
If nets win $100 to win $600, but lose $400 on Spurs ... = +$200 profit
If spurs win $400 to win $200, but lose $100 on Nets ... - +$200 profit.
Hope that helps.