if you touch the 2 big prongs: 1. prong with power wire to battery. and 2. which should just have a small wire going from the solenoid to the starter. This will just run the starter motor, meaning it will spin but it will not kick out to where it will engage with the flywheel.
What you need to do is touch the #1 prong with the smaller prong that has the wire going to the ignition, that gives power directly to the starting prong bypassing the actually ignition. I have heard that if you disconnect the small wire that goes to the ignition, and then connect the #1 prong to it, it will turn the car over then, but i havent tested that.
I dont really know what you think is wrong so i'm gonna add a little more, if when you turn the key and nothing happens, and thats why your trying to do this then what you were orginally doing is the right thing. Connecting those 2 prongs (#1 and the small ignition prong) will tell you if you have a short going from the ignition (key) to the starter.
Connecting the #1 and #2 prong (as stated above) tells you if your starter motor is bad. If it doesnt make a noise and start to spin, its bad. Yes there will be a lot of sparks but just hold it there till you get a good connection! Dont be scared.
Another thing you can do, assuming you have a voltmeter is ground the ground wire on it, and stick the positive wire onto the #2 prong, more specifically on the little amount of wire that goes from the solenoid into the starter, and then have your friend try to start it. It should read 12v. if it doesnt, that means your not getting the right amount of power from the solenoid to the starter and just your solenoid is bad.
And the diagnosis for what you tried to do could be a few things, (as well as maybe why it wont start with the key "assuming it doesnt") but it could either be because the ignition wire was still hooked up to it (like the theory i mentioned earlier) or maybe the starter is just shimed wrong? Which would be preventing the gear to engage to the flywheel correctly and fully start it. Idk, just some suggestions
I hope this helps, like i said i didnt know exactly what you were trying to fix so if i gave extra unneeded information im sorry. But thats basically everything you can do to test your starter so.