Whether avast! has flash scan capability as in mbam or not…?
Flash as removable media?
Flash as Internet content?
First is for File Shield and second for Web\Net\Script Shields.
From an Administrator at the Malwarebytes Forum
The flash scan is supposed to be quicker than a Quick Scan. We principally introduced it for our paid users, who can trigger our Scheduler to run a flash scan after every database update. The idea of the flash scan is to give a quick-and-dirty picture of whether there is likely any malware at all on the system, and if there is, the user is recommended to follow up with a Quick Scan.
http://forums.malwarebytes.org/index.php?showtopic=45105
SAS now has both a First Chance Prevention and a Critical Point Scan. The first on boot up and the second manually or through their scheduler.
First Chance Prevention SUPERAntiSpyware examines over 50 critical points of your system each time your system starts up and shuts down to eliminate threats before they have a chance to infect and infiltrate your system.
http://www.superantispyware.com/superantispyware.html
First Chance Prevention scans critical locations of your computer on startup/shutdown/login/logout. Critical Point Scan is a directed scan that will scan the First Chance Prevention points in addition to memory and loaded modules.
http://forums.superantispyware.com/index.php?/topic/4267-first-chance-prevention/
Both SAS scans i.e. First Chance Prevention and Critical Point Scan are pretty quick. First Chance Prevention is quicker because it doesn’t scan loaded modules and memory. I just ran a Critical Point Scan and it took 31 seconds. Your computer’s scan times will most likely be different 
I liked the ideas:
- A instantaneous scan most probably just of the new added items in the database. The problem is “what is being scanned” so fast imho.
- A dedicated module/default scanning for critical parts.
Look at a question and the reply to it from a MBAM moderator,
What is Flash scan in MBAM PRO ? It says flash scan will analyze memory and auto run objects...., memory means RAM...?
Yes, that is correct. Memory is RAM, in other words, the processes running in memory. It also checks common startup locations (autorun) as well as performing a quick heuristics check to look for new and unknown infections. The only part of the scan that it skips in comparison to the Quick Scan is the scan of files and folders where malware is known to install. The Flash Scan is essentially just a quick check for active infections, and if it finds anything, we recommend performing a Quick Scan so that all locations are checked.
By Harikrishnan’s posts I can see my first assumption is completely wrong.
In other way, although, scan processes already loaded into memory is… well, lose of time… the infection is already running at that time…