Disclaimer: I have never ridden clipless, although I did have Christophe toe-clips and straps on my old upright. Most of us 'horizontalists' who use clipless do so not only for the usual reasons cited by 'uprightists', but also to avoid the dreaded 'leg-suck' phenomenon which can menace riders of recumbent trikes and some of the lower two-wheelers.
As to the effect on pedalling, I think the recumbent experience may have something to tell all cyclists on pedalling technique: first of all, the recumbent posture eliminates the possibility of using your bodyweight on the pedals which simplifies the equation somewhat. Now, my particular bent has a rather less than rigid frame. It has other advantages, so I don't mind, but power transfer is not great. Add in that, bendy frame or not, one can put massive amounts of power down by pushing back against the seat. With those factors in mind consider the following experience: when trying to accelerate in a straight line, if I consciously push on the power stroke, acceleration does ensue, but it is slow and grudging. If, on the other hand, I don't try to consciously push, but just focus on 'unweighting' the non-drive stroke foot, the old dear surges forward as if fitted with an electric motor. From this I conclude that pushing, creates a spiky, uneven power-curve which flexes the frame, delaying power transfer, and incurring frictional losses. Unweighting, on the other hand simply stops your feet from fighting each other, and allows a normal, smooth power curve to have its full effect.
I propose that for the vast majority of cyclists, this is a much more effective way to think about good pedalling technique than stuff about 'pedalling in circles'.
I remain agnostic on clipless, and have done many hundreds of happy bent-miles on flats. I hate wearing any kind of shoes in 'real life', and hard-soled cycling shoes look to me like medieval instruments of torture!:-)
Happy riding, and keep unweighting the non-drive side!
