Under what rules can a get be forced and still be kosher? There seems to be contradiction. On the one hand, a get 'meusa' is posul; on the other hand, bais din can force him.
Here is a treatment of the subject. Basically it comes down to three views:
1) If the Torah requires the get, his real desire is to give it, and the coercing removes the Yetzer Hara's objection, which is not him.
2) It is like someone forced to sell something under duress where they get the payment. Either the "seller" (the husband) loses nothing in the transaction, or he even gains the Mitzvah. Such a transaction, even when forced, is ultimately valid. In other words, the "meusa" has to be relevant to the interests of the husband. If he is just being obstinate, that can't hold the wife as an aguna.
3) Since the marriage was on condition of the Chachamim's agreement (according to the Laws of Moshe and Yisroel - Yisroel being the Chachamim), here the Chachamim just invalidate the marriage. The forced get is simply technique for invoking their rejection of the marriage. The get doesn't accomplish its Torah purpose.
Please see the article for more in-depth discussion of the sources. The different opinions create different circumstances under which force would create a valid divorce.