Use a explod sctrl. in the character's move state. (To find out what state you're looking for hit ctrl+d to enable debug mode and look in the lower-left corner.) Example:
; Stand Light Punch
; CNS difficulty: easy
[Statedef 200]
type = S ;State-type: S-stand, C-crouch, A-air, L-liedown
movetype= A ;Move-type: A-attack, I-idle, H-gethit
physics = S ;Physics: S-stand, C-crouch, A-air
juggle = 1 ;Number of air juggle points move takes
;Commonly-used controllers:
velset = 0,0 ;Set velocity (x,y) (Def: no change)
ctrl = 0 ;Set ctrl (Def: no change)
anim = 200 ;Change animation (Def: no change)
poweradd = 20 ;Power to add (Def: 0)
sprpriority = 2
[State 200, 1]
type = HitDef
trigger1 = AnimElem = 3
attr = S, NA ;Attribute: Standing, Normal Attack
damage = 23, 0 ;Damage that move inflicts, guard damage
animtype = Light ;Animation type: Light, Medium, Heavy, Back (def: Light)
guardflag = MA ;Flags on how move is to be guarded against
hitflag = MAF ;Flags of conditions that move can hit
priority = 3, Hit ;Attack priority: 0 (least) to 7 (most), 4 default
;Hit/Miss/Dodge type (Def: Hit)
pausetime = 8, 8 ;Time attacker pauses, time opponent shakes
sparkno = 0 ;Spark anim no (Def: set above)
sparkxy = -10, -76 ;X-offset for the "hit spark" rel. to p2,
;Y-offset for the spark rel. to p1
hitsound = 5, 0 ;Sound to play on hit
guardsound = 6, 0 ;Sound to play on guard
ground.type = High ;Type: High, Low, Trip (def: Normal)
ground.slidetime = 5 ;Time that the opponent slides back
ground.hittime = 10 ;Time opponent is in hit state
ground.velocity = -4 ;Velocity at which opponent is pushed
airguard.velocity = -1.9,-.8 ;Guard velocity in air (def: (air.xvel*1.5, air.yvel/2))
air.type = High ;Type: High, Low, Trip (def: same as ground.type)
air.velocity = -1.4,-3 ;X-velocity at which opponent is pushed,
;Y-velocity at which opponent is pushed
air.hittime = 12 ;Time before opponent regains control in air
[State 1000, Wood 2]
type = Explod
trigger1 = AnimElem = 2 <----tells mugen to activate this effect on the 2nd frame of state 200's animation.
anim = 192 <---tells mugen to use animation #192
postype = p1 <----tells mugen where to place the effect (E.g. player1)
pos = 60, -70 <----Sets the coords for the effect to appear. (X = 60 pixels Y = -70 pixels)
removetime = 35 <----tells mugen to remove the effect after 35 ticks.