SBPx.m is a function that estimates the onset of impulsive signals, e.g. earthquake recordings. The algorithm computes a simple STA/LTA pick as a rough initial estimate and then evaluates candidate picks around the initial estimate by optimising a ratio of integrated weighted amplitudes from before and after the candidate pick. A more detailed description can be found in the file SBPx_description.pdf.
The function call_SBPx_picker.m shows an animation of what the algorithm does with an example waveform.

The algorithm is called Suspension Bridge Picker (SBPx) because the weighting function resembles a pillar of a suspension bridge.