I've received a painless nikkyo - it is quite bizzare. It is a leading down motion (as nage moves backwards). It is painless in that you are being led downwards without pain - but if you do try and stand up it is painful.
I do like the yoshinkan version of nikkyo as it reduces the chances of getting hit by uke with the other hand - however it can be more difficult to control and put on (especially if your uke is strong or inflexible).
With the standard version (gyaku hanmi):
1. raise both hands together (using grabbed hand to turn uke's blade of hand upwards - and using other hand to pin it to your chest).
2. there is a small hollow below your collar bone - place the back of ukes wrist in there (ukes thumb down and fingers pointing away from your centre).
3. The key is to hold this wrist tightly into your body so your body puts the technique on.
4. Lightly rest your arm (one which was grabbed but now released), on the elbow of uke to keep their arm level (don't force down with this at all, just stop them raising their elbow).
5. Now lean in slightly towards ukes centre (maybe just a bit beyond it). If the contact of the back of ukes wrist is maintained, pain should ensue.
(P.S. if you've got a 6th Dan instructor - he should be able to explain far better! If he is too busy, ask one of the higher grades - nikkyo is very much a feel thing)