filters - Zero-lag butterworth filtering

Recently, I've read several papers where zero-lag Butterworth filters are used (both low-pass and high-pass).

I have a fair understanding of (Butterworth) filters, but how can we design one with zero lag? Can someone explain me how this works?

From my understanding, any low-pass filter yields an intrinsic delay.

Or does this involve bidirectional filtering?

Answer

As you suspect, this paper says that zero-lag Butterworth filtering is obtained by passing the signal through the filter twice: once in the forwards direction and once in reverse.