Locking elbows or not?

I see alot of guys in the gym doing what looks like an exercise to just pump the muscle where they're not going through a full range of motion. For example, on bench they'll lift the bar maybe 8 inches off their chest and back down again and go through that motion quickly. What is that supposed to do for the muscle?
Are there benefits to doing an exercise that way?

I'd also like to know if it's better to not lock out your elbows on an exercise such as bench press. Is it better to keep a constant strain on the muscle completely throughout a set?

Both ways are effective, the non-lockout method is good for filling your muscles up with blood rather quickly... It's all around a better idea to keep strain on muscles throughout the range of motion... Try to keep it strict and squeeze at the top of every rep...

yea and plus if you lock out your elbow at top of the move you put a lot of extra stress on your triceps. So just like a general thought for chest but if you do something like tricep extensions then its god to lock em out because it gives at extra squeez on top. heh any questions?