There's no trigger on these hitch heads. They are positive locking by force. In order to close them you need to push very hard on the back of the jaws. It takes a lot of pressure to lock them manually and I've never been able to do it as both jaws need to be pushed at the same time.
There is an easy way if you are just wanting to test. You could remove the spring that holds the jaws open and then you'll be able to close them with your hands and then the block will slide into place. The spring is located on the underside of the head at the back of the jaws. Just don't forget to put it back in correctly if you do this.
For the poster that said they had to push the lever in to lock the hitch. There's something wrong if you have to do this to get it locked. This hitch should automatically close and lock the lever to the 8 o'clock position when you back into it with the lever set in hitching position at 9 o'clock.
