A Junk Defense is any defense that doesn't conform to standard zone or man defensive strategies. A junk defense can be transitional (typically switching from a zone to man as the point progresses) or some sort of hybrid. The best known junk defense is The Clam.
Transitional junk defense often starts with a zone to defend against set plays off the pull and eventually transitions to man to better protect the endzone. The switch in strategy is triggered by an event - example events are
- set number of passes
- first foul/call
- position of the disc (ie: if/when the offense gets the disc to halfway)
- communication from a designated defender
Once an offense is wise to a transitional defense, it is a good idea for the defense to setup fake transitions to keep the offense guessing. An accomplished transitional defense might involve more than just zone and man, there could be shifting variations of hybrid defense and/or positional defense (eg: the sideline trap).
Hybrid junk defense involves some defenders playing man with others playing zone. 'The Clam' is an example of a hybrid defense. Anecdotally the 2 or 3 defenders playing man mark the handlers while the rest cover the cutting lanes.