Half-time isn’t a tempo change. You keep the tempo (say 140 BPM) but move the snare to land on beat 3 only instead of 2 and 4. Suddenly the groove feels like it’s moving at 70, while the hats keep skipping along at full speed.
It’s the defining move of dubstep, trap and a lot of modern bass music — heavy, spacious and slow-feeling without ever slowing the clock. The contrast between the lazy backbeat and busy hats is the whole trick.