It reads

R commit: after you take damage from an unblocked attack, commit 1 foundation and discard 1 of your opponents momentum.

Does this card allow me to tap one of my opponents foundations and discard one of their mo- or does it make me tap 1 of my foundations and take away 1 of their mo....

It seems weird that they didn't list the commiting of the other foundation as part of the cost.

You can commit either one of your opponent's foundations or one of your own. The effect does not specify which player is to be affected when it comes to the committal effect.

*STAMP* you choose