Lockdown

The term "lockdown" was coined for an effect which happens when a player is being hit by rapid-fire weapons, like the UT200x Assault Rifle, the Minigun, or the Link Gun. Players under the effect of a lockdown cannot dodge or jump, which greatly reduces their chances to avoid being hit, which in turn keeps them exposed to the the lockdown effect while constantly taking damage.

Origin of Lockdown
The lockdown effect was never intentionally added into the game. It is a side effect from the way a player takes damage. In order to bounce players around when rockets or other things explode next to them, the player's physics mode is changed to "falling" whenever a hit with a set momentum value occurs. Small amounts of momentum, like that of a single Enforcer shot, will not cause any significant movement -- the player lands on the ground again within a split second without noticing that he actually lost contact with the ground.

In that short period where the player doesn't touch the ground, he can, by definition, neither jump nor dodge. Since technically you are in the air, you could perform a double jump or, if near a wall, wall dodge in UT2003 and UT2004. In UT you are a sitting duck, though, since there are no mid-air moves. If you are only hit once in a while, this effect will not be a problem since it's very unlikely you are trying to jump or dodge right at the moment when you are hit. With rapid-fire weapons, however, the time between the hits is small enough to increase the chance for this effect to interfere with your dodging or jumping attempts. A dodging move (i.e. both key presses) must be performed entirely while on the ground, and there's a short delay after landing where a dodge cannot be executed. The rate at which a Minigun or Link Gun applies momentum to the player is high enough to constantly keep the player in this post-landing delay or at least to interrupt the dodging input sequence before the key is pressed a second time.

Even the UT2004 Lightning Gun exhibits lockdown, though the effect is so minor you might not have ever noticed. Getting hit with it applies 1 point of momentum, but it's enough to disrupt your movement -- particularly, it can stop a dodge just as it's about to occur. The UT2004 Shock Rifle also exhibits a form of lockdown, which was eventually dubbed "shock tape" by the community. The momentum of the Shock Rifle is so great that often the player being hit by it doesn't have time to move again before the next shot occurs. This allows for a lockdown effect even though the weapon is anything but rapid-fire.

Avoiding Lockdown
In UT200x, the Shield Gun, as cowardly as it is, is one of the best ways to avoid the effects of lockdown long enough to mount some sort of counter. Otherwise, the only thing you can do is hope the guy misses, you can frag him before he gets you, or you can maneuver somewhere to force a break of his lockdown.

In code, a common approach to avoid the lockdown effect is to turn off the damage momentum for rapid-fire weapons with a mutator. In UT2003 this is done by the custom mutator TTM for the Minigun and the Link Gun. In UT2004 the Minigun's and the Assault Rifle's momentum is disabled by default, but the Link Gun still applies momentum and thus locks down players.