The hotkeys window will allow you to assign custom keypresses to various Cheat Engine functions. For example you could assign a hotkey of CTRL+0 to toggle on and off the speedhack feature. You can clear an assigned hotkey at any time by clicking on the 'clear' button.
This is especially useful for scanning in games that make alt-tabbing in and out very difficult, or have some other strange effect when you are trying to move in and out of it's focus for memory scans.
Make certain that any hotkey sequence you choose will not have unwanted effects in-game though. You wouldn't want to load your previous save for example with F7 while simply wanting to do a 'next scan' :-)