A Memo from Esse Beharmast, USSF Director of Officials (September 18, 2000).  
(Editor's Note -- Bottom Line! Don't ever work a dual system of control when only two refereees show up for the match.)

ARTICLES

The Question.
NF rules make a point that a proper tackle is just that no matter where it comes from; front, side, back, whatever. However as a person with a fairly low game count please help me out. On tackles from behind specifically. If a tackle from behind is attempted, poorly executed and ends up being a trip, how do you draw the line as to whether it is a trip and a DFK or SFP and a two game vacation?

The Answer.
Your observation of the action should enable you to "draw the line." Was the player who performed the tackle:
Careless? Was he attempting to tackle the ball but for poor timing or execution tackle his opponent in a non-violent manner? Did his opponent trip because he made physical contact with feet, legs, or body? Direct Free Kick
Reckless? Did you believe he was targeting his opponent and not the ball, resulting in his tripping his opponent in a non-violent manner? Are you convinced that this was not an attempt to injure his opponent? Direct Free Kick and Caution/Yellow Card
Use excessive force? Do you believe that his intention was to injure the player, or you believe he performed the tackle in a violent manner? Direct Free Kick and Send Off/Red Card

Be aware that not every situation where an opponent falls to the ground or stumbles is a foul. If, for example, the player attempting the tackle completely misses the ball AND his opponent, and the opponent subsequently trips over his outstretched legs or body, generally no foul has been committed. The referee, however, must be certain that the player who attempted the tackle did nothing by means of raising legs, hooking feet, grasping with arms, etc., to cause his opponent to trip.
Also on slide tackles, defender slides in and pushes the ball out to the side, his momentum carries him under the attacker who ends up on his face with both feet caught in the defender. I have heard everything from "once the ball is gone, everyone is on their own" to "tripping after a good tackle is no different than any other trip". A little wisdom from experience would be greatly appreciated.
If, in performing a tackle, (or any other player action, for that matter) and in his follow-through commits what would be a foul under any normal circumstance, a foul has indeed been committed. The referee may, of course, decide that the contact is trifling or may apply advantage if it is the best interests of the offended team and in the referee's opinion will not compromise player- or match-control, or the safety of a player. The well-beaten horse "I got all ball" is all bull. Players want you to allow them to gain possession of the ball by any means, regardless of the result of their actions. Players will love you (when it is to their benefit).
Referees who accept the rather flimsy argument of "once the ball is gone, everyone is on their own" are either very weak in foul identification or in courage. Referees cannot fulfill their duty to protect players from injury caused by illegal actions if they cannot or will not sanction (either immediately or at the next stoppage) such actions.
Referees must be careful to be certain that an actual foul has occurred if they adhere to the thought that "tripping after a good tackle is no different than any other trip." Be certain the trip was a result of the player's actions (physical contact caused when the speed of the tackle actually carries the player into his opponent's legs, the whip of the player's legs continues into his opponent's legs, or if the player lifts his leg(s) and by lifting them catches his opponent's legs) causing the trip. If one or more of these things have happened, a foul has occurred.
Whether you call it depends upon your decision that the action was careless, reckless, or involved excessive force (violence) and whether advantage is appropriate. In most cases, if you allow advantage it is very wise to be seen at least "discussing" the trip with the player who committed the foul if you do not caution or send them off.