How to make recovering any stuck object (a vehicle, a tree, a boulder...) easier.
IF THE IMAGE IS TOO SMALL, click it.
This works the same if you swap the vehicle & anchor, for self-recovery.
Any flexible tension member (rope, chain, cable, strap...) applies force along its own axis. Pulling an anchor THROUGH the ground (as in the first diagram) is much more difficult than pulling it OUT of the ground (as in the 2nd). So simply bending the recovery line upward near the stuck object will substantially reduce the towing force needed to extract it.
Conversely: if you need an anchor point for self-recovery, the deeper it is in the ground, and the lower the recovery line runs to it, the better an anchor it will be (as the base of a tree or boulder).
It's worth noting that the supporting structure should not be higher than the connection point on the towing vehicle, especially if the recovery line is short because this principle works at BOTH ends of the recovery line. If the line runs upward from the recovery vehicle, the line's force will reduce that vehicle's traction. But if winching out of a hole, straight up is the easiest direction for the winch to pull so it lifts the truck out of the hole.
For more detail, go to p.160 of
FM 21-305. For even more, read
FM 20-22. If those links are dead, Google the FMs.
A heated discussion about strap vs. chain, and recovery points:
http://www.fullsizebronco.com/forum/21-noobie-bronco-tech-questions-flame-free-zone/78678-straps-vs-chain-6.html 
