Lockdown

The term "lockdown" was coined for an effect which happens when a player is being hit by repeat 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 is changed to "falling" whenever a hit occurs. Small amounts of momentum, like that of a single Enforcer shot, will not cause any significant movement an the player lands on the ground again within a split second without noticing that he actually lost contact to 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 repeat fire weapons, however, the time between the hits is small enough to increase the chance for this effect to interfer with your dodging or jumping attempt. Also, 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 interupt the dodging input sequence before the key is pressed a second time.

Avoiding Lockdown
A common approach to afoid the lockdown effect is to turn of the damage momentum for repeat fire weapons. 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 diabled by default, but the Link Gun still applies momentum and thus locks down players.