Sometimes, paradoxical though it may sound, constraints can make us more free. The classic example features Odysseus binding himself to the ship's mast so that he might safely listen to the Siren's song. Mundane examples along the same lines might involve self-imposed threats and incentives to encourage our future selves to overcome weak-willed procrastination. In such cases, we sacrifice our freedom at a moment in order to achieve greater freedom overall.
But not all instances of "freeing constraints" involve cross-temporal tradeoffs. Another important class of freedoms may be obtained by relieving an unwanted option from others' expectations. I've previously explained the potentially liberating effects of making cycle helmets compulsory in a society where there had been social pressure against them, for example. A more serious example would be headscarf bans in French schools, which free Muslim girls from culturally imposed (and not always welcome) dress norms.
G.A. Cohen has made similar observations about how constraints can improve the options open to the desperate in bargaining situations. Consider a desert island situation: if my life depends upon your co-operation, then you are in a position to demand anything at all from me in exchange. But if there are constraints on what I am free to give, then these are also constraints on what you can demand from me.
If I can't repay my debts, might I be forced to sell not just my house, but also my kidney? Or might someone be disqualified from unemployment benefits because they haven't tried the "option" of prostituting themselves? (Of course, a total ban may not be the only - or the best - way to protect against these threats. But they serve to illustrate the general point.)