How the split way tool works is dependent on what is selected.

1. One or more NODES (and, optionally, also one way) selected:

(All nodes must be part of the same way. If a way is also selected, that way must contain all selected nodes.)

Way is split at the selected node(s) into contiguous ways. If you select just one node then the way will be split into two ways, if you select two nodes, the way will be split into three ways, etc. If the original contained one or more parts that were not reachable from any of the nodes, they form an extra new way. If there is no existing node at the point where you want to split the way, then you can first add a node using the node insert tool.

2. One or more SEGMENTS (and, optionally, also one way) selected:

(All segments must be part of the same way)

The way is split in a fashion that makes a new way from the selected segments, i.e. the selected segments are removed from the way to form a new one. This function will result in exactly two ways.

If splitting the segments out of the way makes a non-contiguous part from something that was contiguous before, the action is aborted and an error message is shown.

3. Exactly one WAY selected

If the way is contiguous, you will get an error message. If the way is not contiguous it is split it into 2 or more contiguous ways.