Halfswording is not allowed in all Kingdoms, please check local regulations.
This technique is useful in close combat against a sword and shield. In close combat using a normal grip the weak part of a two handed sword is used for parrying, by changing to a halfsword grip you get much stronger and can bind your the opponents sword.
You opponent also opens up on the sword side, so you also has the opportunity to thrust. Be careful if using a face thrust because halfsword techniques are very powerful.