When I had the Ikkyo pin applied to me, at first I thought it was ineffective, but when done properly* and relaxed, kneeling with the tegatanas pinning the arm actually do prevent a person from getting up, a lot of it is being relaxed while the other struggles wildly to get free.
My sensei demonstrated what happens when someone tries to hit you with the other arm, he merely looped his left around mine, rested his knee (I think) in place of the other hand and voila, I was totally helpless.
In my relatively short Aikido experience, if I was applying a pin I may need to resort to more "street-effective" and unorthadox methods if I didn't want to injure the attacker, but I'm confident that with more experience in applying the techniques I would be able to rely mainly upon them instead of falling back on previous experience.
Every lesson I feel I may be getting the hang of a technique, but then I get shown a tiny thing that makes it that little bit more effective. I was also told that out there, whatever you can do to pin them, do it, don't try too hard to make a specific pin work or else it'll be too late.
If in any doubts, ask your sensei. He may, as Daniel found out, demonstrate a non-training pin.
*I say properly only in the sense of the way I personally have been shown how to perform the pin, and this in no way reflects changes in methods that may occur from club to club.