It's what you expect to see - and might have seen - on a football field. There's no note stashed away in the pocket, but generally a message, a protest, a statement, scrawled on a white t-shirt under the player's team jersey. 'Take that!' some say. Others make political statements. Some show their support for a cause or a player, or their opposition to a cause or a player or even an official or an organisation. Occasionally, someone will pull out a flag or a banner.
The context is obviously Denesh Ramdin's rather rude, and provocative, jibe at Viv Richards after reaching his century on Sunday in the Edgbaston Test. (Read:Ramdin takes jibe at Viv Richards after ton)
Yes, I have seen some cricketers in the past gesticulate in the direction of the commentary/press box if they have been criticised by someone sitting there. Off-hand, I recall Nasser Hussain going slightly crazy after scoring a hundred against India in the NatWest final back in 2002. Kevin Pietersen's done it a couple of times too, as has Yuvraj Singh. Mostly read-my-lips stuff, usually not much more.