What constrains a paraglider vertically?
To be similar to the constraint produced by a rail, this vertical constraint should be in the order of thousands of kg, IMHO. Any suggestions on how to estimate this amount, and its origins?


The thing is that the constraint is something you "perceived":


  I perceived a very strong vertical constraint


not that was actually there.

Only gravity, lift, and drag (and thrust, if powered) are acting on the paraglider, nothing else.

And the lift is easily estimated, as it is roughly equal to the weight of the passengers and the aircraft summed up and being this a paraglider, definitely not in the range of the thousands of kg.

Having this been your first experience, the sensation can easily be explained by your lack of familiarity with the transportation method.