theres a few ways to do a slap:
you can hold your other hand on the conga while you slap, and add a little pressure, this is a muted slap and it has more of a pop in it, but you can also lift your other hand, specially in faster speeds this tends to happen, there is also a third slap, the open slap, in which you let youre hitting hand bounce slightly of the head, and the drum produces a distinct overtone ring.
also there is a slap which is rarely discussed, and it is played mostly on atambaque, its done to the center of the drum, same position as you play tumbao.
and there is the bata slap which produces a specific sound . hope this helps