Here's Buxtorf the Elder's discussion of the Mitzva Tanz in the 1643 German edition of his Synagoga Judaica (Juden-Schul; 1603). From Chapter 28.
Translation from the 1657 English edition:
After these sports and childish fooleries, they fall to feast their bellies in good earnest ; after dinner they dance many Giggs, and Capering Currantos, and when they are about to depart and to put a period to these nuptial sports, dance a dance called Mitzuah, The commandment, or marriage pavia, because it was commanded by God himself.
This they dance in this manner, the chief man at the table takes the Bridegroom by the hand, he another, and so every one his fellow, even so many as have any skill in dancing, takes one another by the hand. So likewise the chief matron and most honourable woman in the company joynes hands with the Bride, and all the rest with one another. Dancing hand in hand and making great noise with their feet, and in this manner put an end to their marriage rites and sports.
Here is a modern translation, which is simply not half as fun:
After this enjoyable entertainment, the real meal is offered. They are happy, and do not think much about Jerusalem. They dance and leap, as everyone who has seen it knows.
At the end of the wedding they hold a dance which they call the Mitzva dance, that is a dance which is held at a wedding because of the law of God. The most eminent person present takes the groom by the hand, and they follow one another. Also the preeminent woman takes the bride, and all women follow, and they all dance round in a large circle. It is a horrible tossing about, and with that they conclude the joyous wedding feast.