The roll up is a basic capturing technique, involving a sacrifice stone. The term has been coined here at SL and we don't currently know of any established name for it.
Notice that every liberty counts in this situation: if there is no white stone at the marked point, is not atari and can capture those two stones instead.
is again a sacrifice stone, meant to reduce Black's liberties. If , then is atari and when connects at , completes the roll up.
Note that the sacrifice is needed: if simply ataris, connects and now White can no longer atari at a, because there is a stone lacking at b.
These techniques often make a big difference at the beginner level.