what effect does jacks mask have on him, bill, Sam, and eric?

From the text:

The twins.......

"Beside the pool his sinewy body held up a mask that drew their eyes and appalled them."

"The mask compelled them."

Bill was frightened.

"He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes."