Dee Jay's primary anti-air weapon since Super Turbo has been his Jackknife special attack (he didn't have it in his first appearance in Super SF2; most people refer to it as "Upkicks" in SF4). While the move can also be used in combos (such as through cancels or as a juggle off an anti-air slide, or even a juggle off of itself), its primary use has always been as a weapon to shut down jumpers. This characteristic is maintained in USF4 and has actually been expanded in some interesting ways.Short Jackknife
The Short version has the least horizontal and vertical range, but also has the most invulnerability and hits only once.
Forward Jackknife
The Forward version hits twice and, although it has slightly less invincibility than the Short version, allows for some excellent juggle opportunities if both hits connect (a second juggling Jackknife) and especially if only the high hit connects (allowing juggles such as a one-hit standing Roundhouse, an EX Max Out or EX Dread Kick, or most dangerously of all, a juggling Sobat Carnival Super for good damage in the corner).
Roundhouse Jackknife
This version is probably the least useful of the three. Although it has the most hits (3), most damage, and most horizontal and vertical range, it also has the least amount of invulnerability, making it easy for jumpers to stuff cleanly. For this reason, you usually only see this version in combos, either off a cancel, or as a juggle off of an earlier Jackknife or EX Hyperfist.
While the Jackknife is a good anti-air weapon, its lack of hitbox underneath makes it a poor choice as a reversal attack against attacking crouchers (it tends to whiff completely). Which is where the EX version of the move comes in . . .
NOTE ON EX VERSION OF JACKKNIFE: the EX version of the Jackknife is a key component of Dee Jay's defense strategy because, not only does it do good damage, it has excellent invulnerability and a hitbox that extends underneath the attack. This makes it a key resource for dealing with opponent's okizeme pressure since it is one of the few tools (his Ultra 2 being the other) that Dee Jay can use to get up close opponents off of him. For this reason, you should always try to have at least one bar of meter at your disposal so that you can let this move rip when you need it and get an opponent to stop pressuring you in close quarters.
TLDR: Always be charging down/back so that you have the Jackknife ever ready and always have a bar of meter saved in case you need to go EX on a bullying opponent.
FINAL NOTE: IMO the Jackknife is one of the most potent special attacks in the SF4 universe; not only for its anti-air properties but for its general use as a tool to shut down opponent's attacks in front of you. The Forward version is excellent at hitting opponents who are either throwing standing attacks or are walking towards you, requiring them to be much more careful in the attacks they throw. The Jackknife is especially good in this respect because it only requires a single direction and a button press to initiate, meaning you can react to incoming opponent attacks more quickly than you could with a dragon punch. The only limiting factor it has is that it requires you to be charging constantly, a strategy which has already been emphasized as a core element of Dee Jay play. Once you become accustomed to charging down/back in all situations, you will be surprised how often you can score the Jackknife in all kinds of useful situations.
2024 UPDATE: Quick reaction Upkicks (AKA "Ume-Upkicks" or a "psychic Jackknife") is a topic I explain in greater detail here.