Substituting without stopping play, as often as you like — but only through your own 4.5-metre-wide substitution zone. Step on too early and you get two minutes.
Handball does not stop for substitutions — and that is exactly where the pace comes from that overwhelms you the first time you watch. You may come on and off at any time and as often as you like, as long as it happens through your own 4.5-metre-wide substitution zone. The price is a strict order of events: the player coming off must have left the zone before the player coming on may enter. Move half a second too early out of eagerness and you have a substitution error — worth two minutes short-handed, which is more than fresh legs earn back in that time. In training the rule is therefore mainly a communication drill: call it, make eye contact, then go. What beginners underestimate: timeout logic is the exact opposite — three team timeouts of 60 seconds, only one per half, and only while your own team has the ball.