What did Mayella do to Tom while he was standing on the chair?

During the trial, Tom Robinson testifies in court (Chapter 19) about what happened between him and Mayella in the Ewell house. She asked him to get up on the chair to help her by getting a box down from the top of a chiffarobe. As soon as he was standing on the chair, Tom continues, Maybella grabbed him around the legs—he says this twice—and frightened him badly by doing so. He then hopped down off the chair, overturning it, and ran out of the house.
The previous day, however, Mayella had testified first. She said Tom approached her from behind, cursed at her and talked "dirt," then put his hands around her neck and choked her. She did not mention Tom climbing up on the chair.