Thats funny. every time i have tried flats i end up pulling my feet of the pedals as i always pull up on the pedals when climbing or accelerating for me its the whole point of being atttached to the pedal.
Technique. The more you use them you’ll stop using SPD techniques for riding on them and your muscles will develop differently also. When I go to flats from a long period without using them my shins hurt.
When using flats is there a little bit of downward pressure on the up stroked required to keep the foot in contact with the pedal?
No. The weight of your leg will hold it there. Your more likely to use the muscle on the upstroke to take pressure off the pedal so you aren’t pushing it up with the leg putting the power in. (Goes for both SPDs and Flats)