Glad to find out this exists.
On my DAW it works exactly like Vic said independent of the grid setting. I had Snap on & the grid set to 8th notes and did alt+Split at the half-note point. All the resulting notes were half-notes not 8ths. Turning Snap off produced the same results. However since I didn’t Split it exactly on the grid line because Snap was off the slight error on my first note propagated across the 8 bars I was splitting so by the end the notes were pretty far from the grid (but all the same length except the last which was short).
This behavior seems the most desirable to me. I’d want to have Snap on so the split is precise. But I don’t want to change the grid value to get different lengths.
Maybe another preference is coming into play?