A gradient that has at least one middle stop between the end stops is called multistage because it contains more than one color transition. In such a gradient, each middle stop has its own color and transparency, but its position is limited to somewhere in the middle between the end stops. In Inkscape, a middle stop is represented by a diamond-shaped handle, as shown in Figure 7 on the color insert.
To add a middle stop in a gradient, double-click or 
-click anywhere on the gradient line. The new stop will automatically get the color and transparency of the clicked point, so that the look of the gradient remains unchanged.
Also, you can drag and drop a color from the palette onto the gradient line. Dropping a color on an existing handle changes the color of that stop; dropping it anywhere else on the gradient line creates a new stop with this color. Also, when two or more adjacent handles are selected, pressing 
adds new stops in the middles of all selected stop intervals (much the same as it does for nodes in the Node tool, 12.5.3 Deleting and Creating Nodes). New handles added by are included in the handle selection, so pressing repeatedly adds more and more handles; if you start with two handles selected and press n times, you will end up with a total of 2n handles.
To delete all selected stops, just press 
. Individual stops (selected or not) can also be deleted by -clicking them.
Deleting is not limited to middle stops; you can delete an end stop as well, so that the nearest intermediate stop becomes the new end stop of the gradient. If you delete an end handle in a linear gradient or a radii handle in an elliptic gradient, the remaining handles do not move, so the gradient span becomes shorter as a result. If you delete the central handle of an elliptic, its nearest handle moves to become the new center. Finally, if you delete an end handle in a two-stop gradient, the gradient disappears and the object gets painted with the flat color and opacity of the last remaining stop.
Pressing 
with some intermediate stops selected simplifies the selected portion of the gradient, removing those stops that can be removed without noticeable change in the look of the gradient (compare 12.3 Simplifying). In particular, new stops created by double-clicking or pressing initially do not change the appearance of the gradient, and simplifying will delete all redundant stops that weren’t moved or repainted since creation. (You may have to press repeatedly to delete all unneeded stops.)
Naturally, a middle stop’s handle can only be dragged or moved with arrow keys along the gradient line no further than its neighboring handles. Dragging a middle handle with 
snaps it to 1/10 fractions of the available range—that is, it will snap to 1/10, 2/10, 3/10, and so on of the span between its neighbors.
Two or more middle stops may coincide. If they have different colors, the gradient in that point will have a sharp color boundary. For example, add two middle stops, paint one green and the other blue, and then drag the green one all the way to the blue one to create a sharp green-blue boundary in the gradient.
Dragging multiple selected handles with 
pressed moves each one by a distance which depends on how close that handle is to the one that is being dragged. The handle which you grab and drag moves all the way, but all other selected handles lag behind, the more so the farther they are from the handle you drag (these distances are calculated using a smooth bell-like curve, similar to the node sculpting feature in the Node tool, 12.5.7.2 Node Sculpting).
Why is this useful? One gradient feature Inkscape (and SVG in general) lacks is profiles, which means you cannot make the transition between colors accelerating or decelerating (i.e., shifted towards one of the two neighboring stops) instead of linear. Yet, -dragging of middle stops makes it easy to approximate such nonlinear profiles. For example, if you have a two-stop gradient that you want to shape according to a curve profile, select both ends of the gradient, press several times to add a number of intermediate handles, then -drag a handle in the middle to smoothly reshape the gradient:
