BPM measures beats per minute. At 120 BPM, a reference beat occurs twice per second. The way musicians feel that pulse, however, depends on style, meter and subdivision.
Why detectors show half or double
A recording may reasonably be interpreted at 70 or 140 BPM. Both grids align with the same underlying pulse, but one usually matches the musical counting better. Count 1, 2, 3, 4 with the song to choose.
Check against a click
Use automatic detection as a starting value and run the metronome with the recording. If the click drifts slowly, adjust one BPM at a time. Human performances may vary and never lock perfectly to a fixed grid.
Practice below target tempo
Choose a speed where you can play with relaxed, repeatable movement. Perform the passage cleanly several times before raising the tempo. Speed built on tension is rarely stable.
Work with subdivisions
Count eighth or sixteenth notes for difficult passages, then return to larger beats. Moving between detailed and broad pulse improves both precision and musical flow.
Build a measurable routine
Write down the BPM where a passage is clean today and choose a small target for the next session. Increases of 2 or 3 BPM are enough to show progress without turning practice into a race.
Once you reach the original tempo, briefly practice slightly above it and then return. That margin can make performance tempo feel easier, provided your movement stays relaxed and articulation remains clear.