And to complement, it goes from +5 to -5 (you might have already noticed that), and it's completely random, you can't control it. However apparently the rider set as 'leader' in the beginning of the stage won't go below -3 (not sure if this is the number).
Also there are three 'states': the first is just an arrow (can either be up, horizontal or down). Then comes a number (with no background color; this is now quite close to his final daily condition) and finally, when the background of the number is colored, that's his final condition. From the 2nd to the last 'state' the variation will be of maximum 2 (positive or negative). Also the bigger the effort a rider does, the quicker you'll know his condition.
Here's the stretch of the manual which talks about this:
Spoiler
Race-day condition
A rider's condition varies according during the season, depending on his training programme, his state of fatigue… However; there are also daily variations within these wider periods. These changes are shown by a white box on the right of the rider's information box. As with "states of grace", a rider doesn't have a precise idea on how he is feeling until he sets out. The more he rides, the better the idea he will have. As a result, Pro Cycling Manager's race-day condition indicator evolves as the race progresses. There are 3 distinct periods.
At the outset, the box contains an arrow which will point in one of 3 directions. Up = feeling good; horizontal = average and down = not so good. These are approximate indicators which will become more precise as the race goes on.
When a rider has a better appreciation of his condition a numeric value (from -5 to +5) will be displayed. This value is an indicator, but it is not final. The number appears on a grey background.
Finally, when things are clear, the numeric value is confirmed and it appears against a coloured background ranging from dark red (-5) to bright green (+5). The passage from phase to phase depends on the effort expended by the rider. The more that is demanded of him early on, the quicker his condition will be confirmed.
