The burst panel is a small panel that pops up at the bottom of the terminal window when you press Ctrl-B. The combo box will allow you to input a new burst or select from a configurable number of bursts that were previously sent. You'll be able to send the burst as many times as you like, either by pressing enter or clicking the "Send" button repeatedly, or by setting the number of repeats in the spinner. The burst panel will be hidden when you press escape.
I'm combining the burst feature with a SWATH-like "quick macro" recording feature. If you press Ctrl-R while the terminal has focus, it will display the burst panel with a blinking "record" icon but keep the focus on the terminal. Any keystrokes sent to the game will also be appended to the combo box. To stop recording, press Ctrl-R again or focus one of the widgets in the burst panel using the mouse. The burst panel will then behave as though it had been shown with Ctrl-B.