Compression vs. undersizing

[Swimmboy]

Compression suits are specifically designed to compress certain muscles for the swimmer, usually the upper thigh muscles. The suits had panels sewn into the fabric that compressed those muscles. Such compression suits are now largely banned. Undersizing can be done in any suit - brief or jammer - that just squeezes everything and can be uncomfortable if the guy has to wear the suit on deck for long periods of time. But swimmers do feel more compact (thus, have less resistance to overcome) in tighter suits.