Every time you hit a PERFECT shot, your Boost meter is increased. Hitting 8 perfect shots will fill your Boost meter and reveal the Boost button.
You can use your Boost at any time during the match. But it might be more effective at certain points in a rally depending on your player's shots. However, you can't build up another Boost shot whilst you're holding on to a Boost. So if you take too long to use your Boost, you reduce the number of Boost shots you could get in the match.
Boosts do not carry over from one match to another.
The Boost shot is always the best possible shot that your player can do in response to the incoming ball. So it might be a drive or a chop or a flip or a smash, depending on the incoming ball and your player's abilities. The resulting shot will use the stats at Power Level 13.
There are some situations where no Boost shot is available. For example, the opponent hits a short chop and then the only response is a basic backspin or topspin shot with little power. In this case, the Boost button will be greyed out. It will return the only shot possible but your Boost will not be used up.
👍 TOP TIP
You can prevent your opponent from using their Boost by playing backspin shots. Unless your opponent has a Flip option, their Boost will be blocked.