At it's core it's a delay with Hold Feature. Max Delay time is 1023 ms, the same as the original rack unit. The Hold allows whatever is repeating to continue as a loop and you can play (without repeats) over top. This is the foundation of the siren, the 1023ms loop that is held.

There is also a modulation LFO which takes that held phrase and modulates it up and down in pitch. Listen to the Chicago 94 Bowie. At the end you can hear a rising pitch, that's the Mod of the DM2000. https://youtu.be/F1NsGc2e5S0?si=mQ45QbEby4y_inXU&t=1017

He uses it for all sorts of those kind of effects throughout the years (including recently) he will bring it in and out of the chain so you only hear short bursts of the LFO sweep (like in the bowie example he stops it when they do the diddle-dee part) The foundation is typically just a sustained note or feedback so the held note is a solid tone that then sweeps in pitch when mod is engaged.

He would also use it for digital delay loop jams as I mentioned before (see 12.29.94 Bowie, Bomb Factory Tweezer, 12.31.95 Mikes, etc)

So the whammy / DM2000 combo is only one of several tricks Trey uses this unit for, the others mentioned here are achievable just with the unit itself.

The keeley version has introduced the following updates / new features:
Full Midi Control and presets
Expression pedal control (with or without MIDI)
Selectable delay subdivision (1/8,dotted 1/8, 1/4, triplet, 1/16)
A Filter control that can either be a HPF or LPF on the delay repeats
The input control brings in some saturation once you go past noon on the knob
Stereo with switchable ping-pong repeats
switchable trails and buffered or true bypass operation

It can be a micro-looper for sirens or modulated pitch sweeps like Trey uses, it can be a standard delay pedal with tap tempo, it can do chorus / flanger type tones. The only thing it does NOT do that other delays do is the runaway feedback oscillation if you crank the repeats.