You should NEVER lock your arms - whether snatching, cleaning or pulling. You're accelerating the bar upward - if the arms are stiff, the only place that bar's momentum can take it is forward - swinging out. The arms are not straight because the elbows are extended - they're straight because you're not bending them. This is a critical point. Let the weight of the bar STRETCH you arms out.
The difference between a pull and a high pull is the weight on the bar only. You can try all you want to high pull 100% of your best snatch, and it won't go very high (unless your snatch really sucks).
So - you want to pull the bar exactly as you would with a snatch or clean, but instead of changing directions at the top of the extension and using the arms to pull you down, you're pulling the bar up with the arms AFTER the legs have extended. Think of it less as pulling the bar up, and more as guiding it - that is, create a path up the body for the bar to travel by keeping the elbows moving out to the sides and up.
There is another pull variation that involves actively pulling the chest down to the bar - that is, the pull is performed like the above, but after full leg/hip extension, the athlete bends the arms to bring the chest down to the bar. This is a way to strengthen that pull of the arms without having to use lighter weights (i.e. those that could be high-pulled). Of course, this style also presents technical drawbacks because the actual 3rd pull will look nothing like this. It's a bit harder to learn and teach in my opinion as well - people will have the tendency to cut the extension short and muscle their way down to the bar rather than focusing on accelerating it up.