Illustrator Help / Enhanced reshape workflows with touch support

Take from http://helpx.adobe.com/illustrator/using/enhanced-reshape-touch-support.html

Reshape segment workflows & touch support

The reshape path segment workflows have been enhanced to support reshaping with touch input on touch-based devices.

In earlier releases of Illustrator, curved path segments could be reshaped using the Direct Selection tool, but their handles were always constrained to their original angles. Now, the reshaping of segments is unconstrained. Also, converting straight segments to curved paths is now possible by simply dragging the segment itself. There are three ways to reshape path segments.

Pen Tool

While using the pen tool, press the Alt / Option key.
When you hover the pointer over a path segment, the cursor changes to the Reshape Segment cursor. 
Dragging a segment will reshape the path segment

A combination of the path segments, reshaped using the Pen tool

A. Original shape B. The Reshape Segement cursor indicates a reshape-ready segement C. When the Reshape Segment cursor is visible, modify the shape of the segment by clicking and dragging. D.Reshape as many segments as required E. Final shape when reshaping tasks are complete 
To continue drawing with the Pen tool, release the Alt / Option key.
To create a semi-circular segment hold the Shift key while reshaping a segment. Holding the Shift modifier constrains the handles to a perpendicular direction, and ensures that the handles are of equal length.

Convert Anchor Point Tool

Select the Convert Anchor Point tool, and then hover the pointer over a path segment.
When the Reshape Segment cursor is displayed, drag the path segment to reshape it.

Artwork modified by reshaping a line segment with the Anchor Point tool

A. Original artwork B. Reshaping a segment with the Anchor Point tool C. Modified shape 
To create a copy of the path segment, press Alt / Option while performing Step 2. 
To create a semi-circular segment hold the Shift key while reshaping a segment. Holding the Shift modifier constrains the handles to a perpendicular direction, and ensures that the handles are of equal length.

Direct Selection Tool

Using the Direct Selection tool on a straight segment moves the segment. However, when used on a curved segment, reshaping is unconstrained, similar to the Convert Anchor point tool or Pen tool. 

Constraining Handles in the Perpendicular Direction

Holding the Shift key while reshaping with the Pen, Convert Anchor Point, and Direct Selection tools constrains the handles in the perpendicular direction. This enables you to reshape curves to semi-circular shapes. This also ensures that the handles are of equal length.