In Massive Attack and Burial's collaboration track Four Walls, the percussion in the middle part of the song does something interesting where it shifts around the beat. It can be heard at 6:22 and again at 6:38. If you tap along to the beat, you will be off after it does one of those shifts. Is there an actual term for this musical effect?
It sounds to me like this:
So, basically a slow but straightforward 6/8 beat, and then around 6.22, the bassdrum breaks the regular pattern - it comes too soon. I'm not sure if I got the exact timing right but it feels like what I denoted - a "hemiole" - the 3/8ths in the second half of the measure are suddenly divided in 2 equal parts.
The hemiole in itself is not strange, and adds a nice bit of tension to the otherwise rather dull 6/8 beat. The strange thing is that the beat then continues as if the hemiole bass drum is the 1st beat of the next, regular 6/8 measure.
To account for the transitional measure, I simply used a 5/8 beat, and then denote the rest as 6/8 again.
I suspect they realized it by simply truncating the original drum loop and appending a new one from the start. But I think this conventional notation is a pretty fair and performable approximation.
The perhaps too technical term is additive rhythms. Even before Messiaen, folk songs (e.g., Celtic, Quebecois, Middle East) also sometimes dropped or added a beat at the end of a phrase, a practice occasionally adopted by post-1970 hymnody, but I know of no universal technical term for that. "Irregular meter" may be as technical as it needs to get.