The Intercept tool allows you to capture multiple proxy streams at the same time and modify the data in transit. Each listening proxy can be configured to actively intercept the data by setting the proxy switch to on. Once interception is enabled, request and responses will appear in the transaction queue without being buffered or displayed in sequence.
There are four available actions depending on what you want to do with the captured data. When you forward the tool sends the modified version of the captured request/response. Dismiss is used to send the original version. There are also "Forward All" and the "Dismiss All" actions buttons which work exactly like their counterparts accept that they apply to all items in the queue instead of just the currently selected one.
The Intercept tool is strictly non-buffering and asynchronous by design. This means that you can edit all captured items and forward all. This feature is essential when testing modern, network heavy applications which tend to produce a lot of traffic using asynchronous loading. With Intercept, this is a non-issue.
If you no longer want to intercept traffic, simply switch to interception mode.