Use the following options to set what happens when you select Curves > Insert Knot.
Select where to insert the knot.
At Selection inserts the knot exactly at the selected curve point.
Between Selections inserts the knot halfway between a pair of selected curve points.
Set to inserts an absolute number of knots according to the Multiplicity value. For example, if Multiplicity is 3 and At Selection is on, three knots will be inserted at the point whether or not it has an existing knot. Multiple knots at any position lets you create a sharp corner there.
Increase by adds extra knots to the point according to the Multiplicity value.
If you turn on Between Selections and set Multiplicity higher than 1, Maya creates multiple knots at each knot, with even spacing between the positions.
Keeps the original curve in addition to creating the curve with new knots.
You can drag a manipulator to reposition a new knot after you insert it. Select the Show Manipulator Tool and turn on Keep Original before using Insert Knot.