Marvel vs Capcom 2/Countering
Countering in Marvel vs Capcom 2 originates from Street Fighter Alpha, and as such is often called an "Alpha Counter". It can also be called a Hyper or Variable Counter.
The motion for an Alpha Counter is QCF+A1 or A2 when your point character is in hit-stun, and requires and consumes one super meter level to execute. This can only be done on the ground. You can extend hit-stun by push-blocking.
When you Alpha Counter, the character associated with that assist button is brought in performing their Counter move as specified by their assist type. This Alpha Counter can either be a normal move or a special move and can be cancelled as usual. Thus a popular Alpha Counter is Cable's Anti-Air Assist Alpha Counter, the Scimitar, which is then cancelled into his Air Hyper Viper Beam (AHVB).
One particular glitch of interest is when you attempt to Alpha Counter into a character that you may not switch into as they were victim of a Snap Back.