Mast Therapeutics, Inc. (MSTX)

[xtreem99]

i'll take a crack at this...
"painting the tape" its called.

they do it to hide an uppward or downward trend, in this case unfortunately a recent downward. the buggars do it to screw up tech. chart readers, who follow daily charts on a certain stock, and also to bump or drop their daily percentage gain or loss to show on the books.
example:
medium level MM taking a big loss for the day (fighting general market flow)will spend some extra money just at the end of day to spike the price to hide his loss. and vice versa. it can be done to hide gains as well, when it dips at end of day the dump shares to drop price to keep the stock off the radar so they can accumulate at a consistant level.

-x