Currently, inputting values into a Binding's MathConverter doesn't immediately apply, giving the impression that it is non-functional.
The values input tend to only come through after the Page is closed and re-opened.

Repro Steps:

1) Create a new Project containing a MainPage.xaml

2) Introduce a first Grid into the Page, with name "TopSkew" and setting its Properties to: Dimension: 512x256 | Vertical Alignment: Top | Background: #FF000000 | RenderTransform/Skew/AngleY: 24

This will result in a black skewed shape at the top-center of the Page.

3) Introduce a second Grid into the Page, with name "BottomSkew" and setting its Properties to: Dimension: 512x256 | Vertical Alignment: Bottom | Background: #FF000000

This will result in a black non-skewed shape at the bottom-centre of the Page.

The goal will now be to apply a RenderTransform/Skew/AngleY value to the 'BottomSkew' shape that is a mirror of the "TopSkew".

4) Select "BottomSkew" and under its Transform/RenderTransform/Skew/AngleY Property, create a Binding to: Element Name: TopSkew | Path: RenderTransform.Children[1].AngleY

At this point, both shapes on the Page should share the exact same skew angle.

5) In the AngleY Binding Menu, apply a MathConverter Converter, and input -{0} as its Expression.
If the TopSkew Skew Angle is "24", this should in theory generate a Skew Angle of "-24" for the selected BottomSkew Element.

Observe how the Value displayed in the BottomSkew's Transform/RenderTransform/Skew/AngleY Property does not include the expected minus, and that the shape on Stage has not changed since editing the MathConverter's Expression.

6) In the same Binding's ConverterParameter field, type "-{0}" (without quotes).
Observe how the BottomSkew shape now has reversed its Skew Angle as expected.

7) Delete the ConverterParameter value input in Step 6).
Observe how nothing has changed on-Stage, or in the AngleY Value.

8) Delete the entire Expression input in Step 5).
Observe how nothing has changed on-Stage, or in the AngleY Value.

9) Save, close, and re-open the Page
Observe how now what is on-Stage reflects the state of the current Properties .

Expected Results: Any Bindings Parameters are immediately reflected both on-Stage, as well as in Bindings where they are used.