Yeah, even though this jewel only stacks 5 times, you renew the total stack every time after the 5th stack until the use time of 20 seconds has run out.  So the faster you stack this on your people, the longer you get a higher bonus added.

Hit use
*1.5 sec cast - +20 stack 10 sec duraction - 1.5 secs used
*1.5 sec cast - +40 stack renewed to 10 sec dur - ~3 sec used
*1.5 sec cast - +60 stack renewed to 10 sec dur - ~4.5 sec used
*1.5 sec cast - +80 stack renewed to 10 sec dur - ~6 sec used
*1.5 sec cast - +100 stack renewed to 10 sec dur - ~7.5 sec used
*1.5 sec cast - +100 stack renewed to 10 sec dur - ~9 sec used

-> and so forth until you hit your 20 second use time, at which the buff finally wears off.  So the faster you apply the charges the longer everyone else has to heal your target at a higher level of buff.