Some weapons are designed to be used two-handed, others one-handed, and a rare few are equally viable either way. To use a weapon two-handed you must assist with the other hand (type ‘ra’ for right assist or ‘la’ for left assist).
In most cases an assisting hand has no techniques of its own other than ‘r’ to release that hand from its two-handed grip. Equally, an assisting hand cannot be used to defend against attacks. However there are benefits to be had from using a weapon in this way - typing ‘l?’ or ‘r?’ for the assisting hand will display the bonuses it grants (+100% divided among Attack, Defence and Damage if the weapon is designed to be used two-handed in melee, +10% to each if not). In addition, using a weapon in this way will generally unlock more techniques, or upgrade the bonuses of existing ones - such techniques will be displayed in cyan in the combat table. Furthermore, your defence cooldown time (see ‘help cooldown’) will be halved, or work at normal speed if you normally couldn’t recover at all (for example, while performing a combat technique).
The exception to the above is bows. A bow can only be used as a basic club when wielded one-handed, but when used two-handed the assisting hand gains access to a selection of ranged attacks. The other hand may continue to make bashing attacks with the bow itself, but this will interfere with any attacks being made with the assisting hand. Like other weapons, a bow can be released from two-handed use via the ‘r’ command. See also ‘help archery’.