In this Ableton Live 12 tutorial from Ableton's Learn Live series, the Fit Notes to Selected Time Range command is demonstrated as a quick way to stretch a group of MIDI notes so their combined boundaries match a chosen selection. The result is that every note's start and end snaps to the edges of the selection, effectively turning a melody or bassline into sustained, overlapping chords in seconds.
The workflow shown here copies a bassline onto a synth track, applies the command via Command+Option+J (Mac) or Control+Alt+J (Windows), then uses the slice-to-grid shortcut to break the stretched result into clean 16th-note hits. That two-step combination, stretch then slice, is a fast way to generate rhythmic chord patterns from existing melodic material without redrawing anything from scratch.