EchoPenguin

Construction mode has been (temporarily) removed while we make it more of a general feature (rather than just the Node tool).   It will re-appear soon.
 
I think the button for Construction snapping was accidentally removed for the last Beta.  It should be back in the next one.  That button opens up all the snapping for handle directions and lengths, and should include the snapping shown in video 2.
 

Like I said, you'd snap your direction first, then perform a second mouse drag while holding Shift to fix the handle length - then you'd have a perfectly symmetrical curve segment.
 
Demonstrated: SnapToReflectedPlusLength.mov
 
It has to be done as two separate actions, else you'd have two independent types of snap that would fight against each other. While it is another step, it means you'll have greater control over the curve.  Since we use Alt to cancel snapping outright, you might otherwise end up fighting against snapped lengths while all you really wanted was to fix the direction.

Ok, Here are some videos to demonstrate all the snapping that I've added for handles.  Please note, I'm not talking about snapping to grid, I am talking about snapping to useful construction angles relative to the opposite curve point and handle.
 
So, the following snaps are shown:
 
1) Snapping to smooth a cusped node - SnapToSmooth.mov
2) Snapping to inline, and 90 degrees to inline - SnapToInlineAndNormal.mov
3) Snapping to reflected angle - SnapToReflected.mov
4) Snapping to parallel direction - SnapToParallel.mov
5) Snapping to 90 degrees of parallel - SnapToParallelNormal.mov
6) Snap to logical triangle (useful for square corners?) - SnapToTriangle.mov
 
The useful thing about these snaps is that they will enable you to perform common curve construction, completely independent of grid or set axis.  You can use them in tandem with a second action while holding Shift to snap the handle lengths to match the preceding or following handle (while maintaining the direction you already snapped).

Ok, here is a selection of videos showing handle snapping.
 
This one shows snapping the node, then constraining the handles in line with the grid axis, and finally snapping the handle lengths to match.
HandleSnapping2.mov 
 
This one shows constraining the handles to diagonals, then snapping the handle lengths.
HandleSnapping3.mov
 
This one shows how you might match up handle lengths between a number of nodes.  Note the different markers to show whether you have matched the length of the partner handle of the owner node (one bar), or the handle at the other end of the curve segment (two bars), or both (showing both one and two bar markers).
HandleSnapping4.mov
 
And, showing the last example constraining the handle direction to grid axis.
HandleSnapping5.mov
 
Note: in none of these examples is "Snap to Grid" turned on.  The grid is only displayed as a visual cue.  The constraining directions are taken from the grid axis directions and intermediates/diagonals.