Microshocks are the phenomenon when a person gets charged in an electric field. When they touch a conducting object they discharge, and although the amount of charge involved is small, because that is concentrated on the small area of the skin where the contact is first made, it produces a sensation very much like the discharge you can sometimes get after walking across a carpet. See more on microshocks in general.
One specific way this can happen is by riding a bicycle underneath a high-voltage power line. If you are in electrical contact with a metal part of the bicycle all the times, then no charge can build up between you and the bicycle, and you should not experience any microshocks. But if you are electrically isolated from the bicycle - e.g. you are holding rubber handlebar grips, or are wearing insulating gloves - then a charge can build up. This can then discharge as a microshock. The commonest place for this to happen is either on the fingers if they brush against the brake lever, or in the inside of the upper thigh, as it comes close to the top of the seat pillar just below the saddle or to the saddle rails once each pedal revolution.
These microshocks do not cause any harm to the body or have any lasting effects that we know of. But in the highest fields - that is, under spans of 400 kV power lines with the lowest clearance - they can be mildly painful, and they are certainly disconcerting because they are usually unexpected. (more on electric field levels under high-voltage power lines and on the sizes of the voltages and charges involved in microshocks)
They are, however, completely avoidable, by the very simple measure of keeping a finger touching the brake lever (or the bare handlebar inboard of the handlebar grips, or a metal bar end, or any other metal part of the bicycle) for as long as you are under the power line. You don't need to grip it tightly, as long as there is a firm contact (if you only lightly brush your finger against the brake lever, you run the risk of getting microshocks at that point, which is the very thing you are trying to avoid).
After dark, the microshocks may be visible as small flashes, less than a millimetre long. Again, this can be disconcerting if you are not expecting it, but there is nothing untoward about it; the energy in the flashes is very low, and it does not indicate any fault with the power line.
If you experience problems on a bicycle under a power line that seem worse than the description here, please let us know as we would like to investigate.