How to Play the Marimba
Various playing techniques

In general, it is most common to perform with two (one in each hand) or four (two in each hand) mallets. However, there are also some people who use six (three in each hand), as well as those who take four in each hand for a total of eight, producing grand, stately tones. The key is to hold the mallets in such a way that the thumbs and index fingers can move freely, in order to adjust the distance between mallets to hit the intended tone plates.

When you want to lengthen a note: on a marimba, whose notes are shorter than those of a wind or string instrument, you repeatedly hit a tone plate, keeping the amount of time that the mallet in contact for any one blow as short as possible. Doing so makes it sound almost exactly like a single note is continuing for a long time. This is tremolo. During a tremolo, a soft mallet is usually employed. In cases such as when a player wants to play the marimba while performing with another instrument using one hand, the player may play the marimba by sandwiching the tone plates up and down between mallets. The glissando, in which the tone plates are rubbed to the left and to the right to produce a continuous sound, is another playing method characteristic of the marimba.
See these videos of performances.