Uke grabs one of tori's wrists with one hand (gyaku-hanmi = mirror stance)
It is in uke's interest to maintain a good - full-palm - grip while keeping arms and shoulders mobile ("sticky grip"). At the same time, a vigilant connection is established via this grip.
Katate-dori is not the goal of the attack, but only the first contact. It enables uke to fix tori for a short moment or to move them into a position where uke can continue the attack.
When only katate-dori is announced, the execution from gyaku-hanmi (mirror stance) is expected. This is one of the most common attacks in Aikido.
Terms used:
Gyaku-Hanmi, Uke, Tori, Aikido
